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.
hmm: I'm a little surprised this works without the delete... We're just updating a solr record, and not changing the id, right? If that's the case, I think you would need to do an `update`, while an `add` like this would fail... but if you've seen it work, then I must be misunderstanding something.