Fixes for bootstrap-sass used without Rails

Two major changes:

1. Improve the load logic to take into account that Sprockets might be used without Rails. Simplify and DRY it up. It should have exactly the same effect as before, with the exception that `rails_functions.rb` is now only loaded if ::Rails is defined.
2. Improve the `image_path` sass function to delegate to the Sprockets `image_path` directly if Sprockets is available. This is a near-direct copy of sprockets-sass' version here: https://github.com/petebrowne/sprockets-sass/blob/master/lib/sprockets/sass/functions.rb#L62 This covers Rails and Sprockets usage, while the next condition for Compass continues to cover Compass-only use. This fixes #257 - SystemStackError infinite loop when bootstrap-sass is included with Sprockets but not Rails.

Overall this pull should make bootstrap-sass work with non-Rails installations that use Sprockets. I have also tested with our Rails app and all continues to work great, since the new `image_path` function is equivalent to the one sprockets-sass provides. There should be no conflict.

I also made a quick Sprockets-Sass-Compass Rack (man that's a lot of buzzwords) test app here. It may be useful: https://github.com/trisweb/compass-test-app