Updates the example app to use CDTReplicators properly.

The CDTTodoReplicator keeps a strong reference to the
CDTReplicator objects that it creates and sets itself as
their delegate. The CDTReplicators are released upon
completion or error.

The polling block has been removed.

The CDTTodoReplicator now uses the new CDTPush/PullReplication
configuration and the CDTReplicatorFactor -oneWay:error method
to build CDTReplicator objects.

Additionally, the CDTAppDelegate now keeps a persistent
CDTTodoReplicator object, which the CDTViewController uses to
fire off synchronization.

Adds a description method to CDTReplicator.