adjust spritmap class to usr load path to get relative sprite names

Gemfile.lock

@@ -1,7 +1,7 @@
 PATH
   remote: .
   specs:
-    compass (0.12.0.alpha.0.5e89865)
+    compass (0.12.0.alpha.0.71494ce)
       chunky_png (~> 1.2)
       fssm (>= 0.2.7)
       sass (~> 3.1)

lib/compass/sass_extensions/sprites/sprite_map.rb

@@ -17,10 +17,18 @@ module Compass
           name, path = Compass::SpriteImporter.path_and_name(uri)
           files = Compass::SpriteImporter.files(uri)
           sprites = files.map do |sprite|
-            sprite.gsub("#{Compass.configuration.images_path}/", "")
+            relative_name(sprite)
           end
           new(sprites, path, name, context, kwargs)
         end
+        
+        def self.relative_name(sprite)
+          Compass.configuration.sprite_load_path.each do |path|
+            if sprite.include?(path)
+              return sprite.gsub("#{path}/", "")
+            end
+          end
+        end
 
         def initialize(sprites, path, name, context, kwargs)
           @image_names = sprites