Added support for the $<sprite-map>-prefix attribute.

With the $<sprite-map>-prefix attribute the default prefix (= sprite folder name) for the generated sprite classes can be overwritten.

My use case: In some cases we use our company branded icons as single images, in other cases they are merged in sprite sheets. It's easier for everyone if file names and sprite classes are identical. So i don't want the sprite classes to be prefixed with their including folder's name. In this case I set the  $<sprite-map>-prefix to null.
(This could also be the use case for Issue https://github.com/chriseppstein/compass/issues/249)

I found the existing $<sprite-map>-prefix attribute in the original code unused. So I hijacked it for my purposes.