#316 Add 'type' document property to the CBLModel

* Add 'type' document property to the CBLModel.

* Add API_ModelTypeProperty test to ModelTests.
I think this should be moved down to the "protected" section, since it's mostly there for subclasses to use. (Code that uses a model object should be relying on the model-factory to create the right class, instead of having to check the type itself.)

Also, the comment should say something like:

The type of document. This is optional, but is commonly used in document databases to distinguish different types of documents. CBLModelFactory can use this property to determine what CBLModel subclass to instantiate for a document.