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.
Do you think we'll use this? We never tried to run it on production. Ive wondered if it should be deleted. If you do plan to run it in the future, do you think the slugging logic is reasonable?