Updated external config for compass

Hi there!

This builds off of #3 thanks to the work from @paschdan, with a few changes:

1) Removes the `add_import_path` that imported bootstrap for simplicity
2) Moves the Compass configuration file
3) Renames the variables in `Gruntfile.js` - unrelated to this really, but I think these new ones are much more descriptive.
4) Adding some more comments

@paschdan - what do you think?

Thanks!