rename ImagesSet to ImageSet, makes more sense
move sass script function inclusion into our SassFunctions module
inherit from Sass' built-in Literal class for our SpriteMap
build a monkey patch to hook into the sprockets manifest compilation method

also has SpriteMap inherit from a Sass literal
full integration tests for Rails 4.0 and 4.1

uses Appraisal for testing against different versions of Rails