Bumps for Homebrew integration

Here it is!  The giant PR for Homebrew integration.  This should allow for compiler and pain-less installation of dependencies on OSX for things such as IJulia and Cairo.  My workaround for https://github.com/JuliaLang/julia/issues/4186 right now is to just error out and ask the using to manually install "Homebrew" for now, pending a resolution.  Right now I think that's better than leaving everything in an inconsistent state.

@StefanKarpinski I'd especially like you to take a look at this to see if there's anything you object to.  [Here is an example](https://github.com/JuliaLang/Cairo.jl/commit/b980baf5151a21e9340d339f19da7e05b4596d2d) of the workaround.