Makes CDTAbstract/Pull/PushReplication conform to NSCopying.

This ensures that the replication configuration cannot change after
the instantiation of the CDTReplicator object, which could cause some
bugs.
Merge pull request #48 from cloudant/32208-make-CDTAbstract-Push-PullReplication-conform-to-NSCopying

Makes CDTAbstract/Pull/PushReplication conform to NSCopying.