Duplicate sprite group support. Closes #10

This is a work in progress.

The goal is to make the transform support duplicate sprite group definitions, which might happen if you include the same sprite sheet in multiple files using SASS `@import` or similar. In this case all sprite group defnitions should be taken into account and the sprite generation should only happen once.

If there are multiple non-identical definitions of the same sprite group it's probably an error, and the transform should skip spriting and warn the user about it.