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.
Please remove "often". This guide explicitly recommends using pip.