Modify setuptools and pip docs for OSX

Clarify a little more what setuptools allows you to do, what `pip` is, and what advantages it has over `easy_install` provided by Setuptools.

This makes the docs sound somewhat opinionated, but simplifies things a little for new users. If we mention Setuptools, `easy_install` should be mentioned at least; and then some information towards why a lot of people will say "just use pip!".

Let me know if some wording should be changed, or if you think the documentation as existing is better.