Updated README with install instructions.
Updated README with install instructions.

The install instructions were moved to near the front of the README, and
redundant instructions were removed from later sections. Further, an appendix
was added that gives examples of installation for specific platforms.
Finally, current lack of Python 3 support was clarified.

Resolves Issues #159 and #191

dcolligan/server,adamnovak/server,srblum/hackathon-server,hjellinek/server,pansapiens/server,adamnovak/server,srblum/server,srblum/hackathon-server,david4096/ga4gh-server,saupchurch/server,shajoezhu/server,ga4gh/server,hjellinek/server,ohsu-computational-biology/server,diekhans/ga4gh-server,ga4gh/server,saupchurch/server,shajoezhu/server,macieksmuga/server,ekalosak/server,ohsu-computational-biology/server,david4096/ga4gh-server,macieksmuga/server,UMMS-Biocore/ga4gh-server,dcolligan/server,ekalosak/server,UMMS-Biocore/ga4gh-server,srblum/server,diekhans/ga4gh-server,pcingola/server,david4096/ga4gh-server,ga4gh/server,ohsu-computational-biology/server,diekhans/ga4gh-server,pcingola/server,srblum/hackathon-server,srblum/server,adamnovak/server,pcingola/server,dcolligan/server,hjellinek/server,pansapiens/server,saupchurch/server,UMMS-Biocore/ga4gh-server,ekalosak/server,macieksmuga/server,pansapiens/server