API: Added CBLReplication.pendingDocumentIDs property

Allows app to see which docs have local changes that haven't been pushed
yet.
For #442
API: Added -[CBLReplication isDocumentPending:]
Merge branch 'master' into feature/unpushed
Merge branch 'master' into feature/unpushed
Merge branch 'master' into feature/unpushed
Merge pull request #557 from couchbase/feature/unpushed

Adds methods to CBLReplication to determine doc IDs of unpushed changes. (For #442)