Remove all init methods from CBLModel API

* Removed all init methods from CBLModel public api.

* Added modelForNewDocumentInDatabase: (CBLDatabase*)database in replacement of the initWithNewDocumentInDatabase: method.

* #521
Move awakeFromInitializer to the end of CBLModel initWithDocument:

Move awakeFromInitializer to the end of CBLModel initWithDocument: so that document data if available can be accessed.
Merge pull request #533 from couchbase/feature/issue_521

Remove all init methods from CBLModel API