Adds methods to get conflicted documents and relevant tests

* TD_Database+Conflicts adds methods for retrieving
and (eventually) manipulating conflicted documents.

* CDTDatastore -getConflictedDocumentIds returns NSArray* of
document ids that have conflicts

* DatastoreConflicts tests the creation of conflicts and their
subsequent retrieval.
  - adds test that cannot create a conflict locally using the standard
CDT* API
  - adds method and test to create a conflicted document using TD* API