Feature/sass flag

Now using `yo ember --sass` will ask if you want to use compass Bootstrap, and include all compass references in the Gruntfile as well as node dependencies.

This addresses issue #98, in it's own way. This generator should only be opinionated concerning Ember standards, and it's a far stretch to assume that everyone using Ember also uses Sass/Compass. Let me know if I missed anything.

Left to do: add general Bootstrap (vanilla css).