Update the OS X development documentation

* Remove some outdated information from the OS X development guide
* Add `scripts/osx_dev` to bootstrap a development environment
* Add a `Brewfile` to track binary dependencies ([more information](http://robots.thoughtbot.com/brewfile-a-gemfile-but-for-homebrew))

I apologize in advance if you don't like documentation pull requests without prior discussion. I couldn't find anything in the contributor guide about documentation (I only found stuff about bugs and features).