Automatically discover homebrew lib/ and include/ paths if installed on OSX.

This change allows Pillow to discover installed libs on OSX when using the homebrew package manager outside of `/usr/local/`.

It relies on the commands module, which goes away in Python 3.  If it seems like a good change to add, I can switch the code over to subprocess.
Merge pull request #330 from donspaulding/patch-1

Automatically discover homebrew include/ and lib/ paths on OSX