Update installation docs to mention about pip.
Update installation docs to mention about pip.

Other changes:

- Minor markup and style changes
- Use https in URLs
- Remove the "python setup.py develop" part from the
  documentation. It's useful for development.

ccl0326/gunicorn,prezi/gunicorn,tejasmanohar/gunicorn,GitHublong/gunicorn,ephes/gunicorn,1stvamp/gunicorn,keakon/gunicorn,mvaled/gunicorn,mvaled/gunicorn,jamesblunt/gunicorn,wong2/gunicorn,zhoucen/gunicorn,malept/gunicorn,z-fork/gunicorn,ccl0326/gunicorn,zhoucen/gunicorn,ccl0326/gunicorn,jamesblunt/gunicorn,gtrdotmcs/gunicorn,gtrdotmcs/gunicorn,wong2/gunicorn,prezi/gunicorn,wong2/gunicorn,beni55/gunicorn,MrKiven/gunicorn,malept/gunicorn,elelianghh/gunicorn,zhoucen/gunicorn,gtrdotmcs/gunicorn,prezi/gunicorn,malept/gunicorn,1stvamp/gunicorn,1stvamp/gunicorn,mvaled/gunicorn,WSDC-NITWarangal/gunicorn,jamesblunt/gunicorn,harrisonfeng/gunicorn,tempbottle/gunicorn