[WIP] Added a Replicator object

This is a work in progress for the Replicator object. It can successfully create a replication -- next steps are to fill out and test the stubs for getting replication status, getting all replications, etc.

Would like some feedback to verify that I'm going in the right direction with this, though.

@ksnavely @evansde77 @jcounts @dmannarino 
@evansde77 Yes, but not marked as a private method. Want me to just get rid of the underscore in front of _database_url? (pylint will complain if we get too free with our use of private methods outside of a class).