Add support for building and running on OS X.

Includes the necessary code changes, CMake changes, and instructions to get your own copy of Sigma running on your Mac.  Huzzah!

OS X does not require glew to (sanely) use OpenGL.  OpenGL calls just work.  Both of the extensions Sigma is checking for using glew are Core in GL 3.2, which is the lowest version of GL we can target on OS X.  I've therefore pulled out all references to glew in the OS X build.

All changes include explanatory comments.