Added Bundler support

This pull-request adds [Bundler](http://bundler.io/) support. I use Bundler as it allows me to use different version gems for different projects. Setting the `bundler` option to `true`, it will use the equivalent of `bundle exec` to compile the Sass files.

I may need some guidance on adding a test case however. I couldn't work out if all the test cases use the same config settings or whether I could add a specific set of options just for the Bundler test case. Also to implement this test case it would require you to include a GemFile, GemFile.lock and have the Bundler gem installed. So I'm not sure if you want all those extra dependencies on the test cases.