DOC: add dev environment creation details to contributing.rst
- provides instructions for creating a development on any platform with conda
- cleans up the install.rst a bit

cc @rockg
@jorisvandenbossche 
@shoyer 
@cpcloud 
 very nice, thanks!

I used a very similar procedure myself to setup my dev environment.
 yep, this is one of the current pain points in getting started (esp on windows)
 @jorisvandenbossche just wanted to get this in, if you have comments, ill do a follow up
