(#67) - Add proxy to a remote db support

Requires pouchdb/express-pouchdb#156 to be merged first.

The proxy feature is still new: the core db stuff + fauxton (probably the most important use case) works. /_config still shows the local configuration data though (and I'm not sure if that should actually change).

Also the plug-ins behind _users, _replicator and show/list/update currently mostly assume that an http database means couchdb; they might not work correctly when the backend is something else like, say, pouchdb-express-router, or one of the couchbase things. That's something that can be improved on mostly in the plug-ins themselves, though.