This commit address comments/discussion surrounding the CDT*Replication
classes. In particular, these classes more resemble the similar implementations
in the Andriod library.

An NSError** was added to the signature for the method -dictionaryForReplicatorDocument.

CDTReplicationTests was added to verify the behavior of the CDT*Replication classes.

This commit, along with the previous two, will be squashed to a single commit
when merged into master.