#316 Add 'type' document property to the CBLModel

* Add 'type' document property to the CBLModel.

* Add API_ModelTypeProperty test to ModelTests.
#316 Move the property to the protected section

Move the property to the protected section and improve comment per code review.
Merge pull request #455 from couchbase/feature/issue_316

#316 Add 'type' document property to the CBLModel