Changes slug setter to put slugs on a solr document field named 'slug', ...

...instead of 'id', and leaves the 'id' field alone.

Removes the addition of a 'slug' datastream on Fedora objects. It's not being used.
Haven't decided yet. Postponing removal until i'm fully convinced we don't need it. The logic around building the slug looks fine, and can be easily changed if needed. I like the fact that it lives in `SlugSetter` though.. good design.