This allows for test compilation (which doesn't use sass-rails *or* compass) and could be a lead-in to a fully generic version for non-compass/rails use of bootstrap-sass.
We need a new compass function to handle image-path, since image-url (rightfully) includes the url() wrapper. We don't want the url() wrapper around our path variable, since bootstrap already includes it. This way we only have to patch the _variables.scss file, and not _sprites.scss too.
