(#108) - Adds _replicator database

Adds the ability to persist replications over server restarts.

It's not a perfect clone of the CouchDB replicator yet (create_target only works on http dbs in PouchDB & the retry on error logic isn't the same), but it brings us to the '90%' compatibility level. It should do for now for most use cases. And looking into those two issues is on my todo-list for that plug-in, meaning that it'll probably just work some day without further changes.