Bloated support for OSX

Used Hoard's (Heap-Layer's actually) method of swapping out OSX memory
functions to resolve https://github.com/sholsapp/gallocy/issues/1.

I say this provides "bloated" support because Hoard's method actually is
fairly heavy handed and replaces, I think, too much. We can and should
adjust this implementation to be like
http://google-perftools.googlecode.com/svn/tags/google-perftools-1.8/src/libc_override_osx.h.