Adding Gemfile and bundle

This adds a `Gemfile` for specifying compass and sass versions.

Running `npm install` will also run `bundle install`.

Running the `grunt compass` tasks will use `bundle exec compass` rather than `compass`.

This should fix an issue where if newer versions of `compass` and `sass` are installed, projects written against older versions will still be able to build.

Please test this out if you have time.

```sh
git clone git@github.com:ff0000/generator-red-django.git
cd generator-red-django
npm link
cd ..
mkdir test-red-django
cd test-red-django
yo red-django
grunt build
```