Implement a bundler setup

Using bundler for our gem dependencies will allow us to have project specific gems.

All you have to do is run `bundle` inside of that directory, and the bundle config is set to install the Gemfile gems within a local directory (./.gems) so that the gems are only accessible inside of the project directory. This will prevent project gems from messing with your global gems.

Grunt is set up to automatically use the bundled gems, so you can use it with just the standard `grunt` command, but you can still use command line compass, sass, etc. tools yourself. You'll just run `bin/comass` or `bin/sass` to pick up the bundled gem versions; these executables are generated by bundler.