Reorganize loader for compass support

::Sprockets is the wrong check, since we reference the ::Rails constant directly in the loaded file. This was outright failing when used with an empty compass-sprockets-sass install (without Rails)
Provide image_path that works with Sprockets

Prior to this, functions in sprockets-sass conflicted with ours and caused an infinite loop and StackError. This fixes thomas-mcdonald/bootstrap-sass#257

Note that Sprockets' asset_path requires configuration if it's not already configured by your framework. For example, in a simple Rack app, add the following to your config.ru:

environment.context_class.class_eval do
  def asset_path(path, options = {})
    "/assets/#{path}"
  end
end
Merge pull request #318 from trisweb/tristan_compass_fixes

Fixes for bootstrap-sass used without Rails