Rework setuptools and pip docs

Clarify a little more what setuptools allows you to do, what pip is, and
what advantages it has.
Incorporate changes suggested by @sigmavirus24
Merge pull request #466 from mandarg/master

Modify setuptools and pip docs for OSX
Added Github Atom to env.rst
Merge pull request #473 from syndbg/master

Added Github Atom to env.rst