Add download_python function to reduce repetition
Add download_python function to reduce repetition

Create a common download_python() function that allows most other files in the build/runtimes directory to be simplified to something of the form:

```
#!/usr/bin/env bash
# Build Path: /app/.heroku/python/
# Build Deps: libraries/sqlite

OUT_PREFIX=$1

source ./download_python
download_python "2.7.8"
cd src

./configure --prefix=$OUT_PREFIX
make
make install
```
samuelhavron/heroku-buildpack-python,Scalingo/python-buildpack,thenovices/heroku-buildpack-scipy,ekkleesia2/heroku-buildpack-python,blazekadam/open2,swanson/heroku-buildpack-python-ortools,tommyvn/heroku-buildpack-python,agriffis/heroku-buildpack-python,HousekeepLtd/heroku-buildpack-python,pkumar-appd/python-buildpack,markine/heroku-buildpack-python,joshowen/heroku-buildpack-python,martingascon/heroku-buildpack-python,gaumire/heroku-buildpack-python,pebble/cloudpebble-buildpack,joshisa/python-buildpack,rollokb/heroku-buildpack-python,edmorley/heroku-buildpack-python,arnaudlimbourg/heroku-buildpack-python,nwkotto/geodjango-buildpack-with-geos,YoApp/heroku-buildpack-python-twemproxy,Mause/heroku-buildpack-python,ojacobson/heroku-buildpack-python,liyanchang/cloudfoundary-buildbot-buildpack,adamroth/heroku-buildpack-python-pdftk,predata/heroku-buildpack-scipy,loanzen/heroku-buildpack-python,mobify/heroku-buildpack-python-swig,rbtying/heroku-buildpack-python,AltSchool/heroku-buildpack-python,TipWander/heroku-buildpack-geodjango,andrespires/python-buildpack,nemesiscodex/heroku-buildpack-python,marcospy/heroku-buildpack-scipy,doda/heroku-buildpack-python,monological/heroku-buildpack-python-opencv,joshisa/python-buildpack,pwdel1/heroku-buildpack-scipy,river-jade/heroku-buildpack-scipy,hsr-ba-fs15-dat/heroku-buildpack,joshisa/python-buildpack,ianyh/heroku-buildpack-python-bower,18F/python-buildpack,projetoeureka/heroku-buildpack-scipy,buildbot/buildbot-master-buildpack,resmio/heroku-buildpack-geodjango,bobsilverberg/heroku-buildpack-python-peep,benmurden/heroku-buildpack-python-ember,driftyco/heroku-buildpack-sphinx,benjaminfox/heroku-buildpack-python-geoip,dulaccc/heroku-buildpack-geodjango,miraculixx/heroku-buildpack-python,joshfriend/heroku-buildpack-python,marianzange/heroku-buildpack-python,plangrid/heroku-buildpack-python,samuelhavron/heroku-buildpack-python,Stibbons/heroku-buildpack-python-installer,icoxfog417/heroku-buildpack-python,zakdoek/heroku-buildpack-django,jimlindeman/python-buildpack-collectd,cloudfoundry/python-buildpack,liyanchang/buildbot-master-buildpack,Apogaea/heroku-buildpack-python,doda/heroku-buildpack-python,jamarparris/heroku-buildpack-python-swig,papaeye/heroku-buildpack-python-bower,pkumar-appd/python-buildpack,dvreed/heroku-buildpack-python-ldap,kowito/python-buildpack,ryanhiebert/heroku-buildpack-python,Nazanin84/python-buildpack,RobertSundling/heroku-buildpack-python-nginx,dubsmash/heroku-buildpack-python,slorg1/heroku-buildpack-python,samuelhavron/heroku-buildpack-python,jimlindeman/python-buildpack-collectd,brianthomas/heroku-buildpack-textmining,nadirizr/heroku-buildpack-python-gulp,cdht/heroku-buildpack-python,samuelhavron/heroku-buildpack-python,cloudfoundry/python-buildpack,zakdoek/heroku-buildpack-django,scrapinghub/scrapinghub-buildpack-scrapy,ywarezk/heroku-buildpack-geodjango,wbrefvem/heroku-buildpack-python,ruscoder/heroku-buildpack-python,maxcountryman/heroku-buildpack-python,nicktimko/heroku-buildpack-scipy,samuelhavron/heroku-buildpack-python,skilljar/heroku-buildpack-python,ad-m/heroku-buildpack-python,xica/heroku-buildpack-scipy,liyanchang/cloudfoundary-buildbot-buildpack,ephemerallabs/heroku-buildpack-python,alexflanagan/operand-buildpack-python,kvarga/heroku-buildpack-python,marcospy/hackathon-globo-app,maciejgryka/heroku-buildpack-spcedge,Nazanin84/python-buildpack,wbrefvem/heroku-buildpack-python,vlidholt/heroku-buildpack-scipy,jimlindeman/python-buildpack-collectd,edenspiekermann-amsterdam/heroku-buildpack-python,swils/heroku-buildpack-python,AltSchool/heroku-buildpack-python,kowito/python-buildpack,alexflanagan/operand-buildpack-python,cortexintel/heroku-buildpack-scipy,amanjain/heroku-buildpack-python-with-django-bower,jvenezia/heroku-buildpack-python,gitricko/apt-python-buildpack,gitricko/apt-python-buildpack,rapilabs/heroku-buildpack-python,brightforme/heroku-buildpack-python,oberlin/heroku-buildpack-python,DylanLukes/heroku-buildpack-python,smalleats/heroku-buildpack-python,samuelhavron/heroku-buildpack-python,benmurden/heroku-buildpack-python-ember,cloudfoundry/python-buildpack,marcospy/hackathon-globo-app,mythmon/heroku-buildpack-python-nodejs,soapril/heroku-buildpack-python,pkumar-appd/python-buildpack,samuelhavron/heroku-buildpack-python,cclauss/heroku-buildpack-python,pmclanahan/heroku-buildpack-python-peep,heroku/heroku-buildpack-python,18F/python-buildpack,stalnacke/heroku-buildpack-python,HanSooloo/heroku-buildpack-python,andrespires/python-buildpack,grivescorbett/heroku-buildpack-python,18F/python-buildpack,manufacturedba/heroku-buildpack-python,Brogency/heroku-buildpack-python-gulp,integricho/heroku-buildpack-python-ffmpeg2.1-with-codecs,PerceptumNL/heroku-buildpack-python,andrespires/python-buildpack,beni55/heroku-buildpack-python,samuelhavron/heroku-buildpack-python,Tiger66639/buildbot-master-buildpack,lukaszkorecki/heroku-buildpack-python,kowito/python-buildpack,nikita-pivovarov/heroku-buildpack-python-ldap,Scalingo/python-buildpack,gitricko/apt-python-buildpack,Nazanin84/python-buildpack,igroff/heroku-buildpack-python,ad-m/heroku-buildpack-python,marcospy/hackathon-globo-app,heroku/heroku-buildpack-python,WonderThink/heroku-buildpack-python-ffmpeg-geoip,benjaminfox/heroku-buildpack-python-geoip,junyoed/heroku-buildpack-python,liyanchang/cloudfoundary-buildbot-buildpack,andela-brotich/wger-buildpack-python