Add Gemfile when Sass & Compass is selected

The generator creates a Gemfile and runs `bundle install` to ensure Sass
and Compass are installed (if Sass & Compass option is selected).

I also updated the tests to check whether the Gemfile is created, but
test seem kinda weird. I never tested before, but it seems they don't
test all combinations and the file is named `test.js` instead of
`test.spec.js`, which would be more suitable for jasmine-node. Also,
when the tests fail, the testing just stops, it does tell you how many
tests you ran and how many of them failed. Plus, the output says that
there are 0 assertions, which is not true, and that's because the tests
aren't using Jasmine's assertions. Am I running the tests wrong, or
should I open a new issue?

There's also a `bootstrap-sass` gem, but I didn't really know what to do
with it concerning Glyphicons, so I just left it out of the Gemfile and
decided to stick with Bower.