[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 
same as the _database_url property?