Teach replicator to gracefully reconnect

This patch allows the replicator to recognize the "last_seq" field at
the end of a continuous _changes feed and reconnect to the feed using
that value.

BugzID: 17502
Merge pull request #17 from cloudant/17502-replicator-graceful-reconnect

Teach replicator to gracefully reconnect

BugzID: 17502