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.
so, this is me not knowing how Solr works very well... but I don't think `update` means the same in the context of solr, as it does in, say, a relational db. In any event, using `#add` seems to effectively update the docs, while `#update` has a clunkier syntax, and I'm not sure what it really does. See https://github.com/rsolr/rsolr#updating-solr