Return the row Id of the newly inserted row from Model.save

Model.save relies on SQLiteDatabase.insert to perform the actual insert of a transient Model. Since the insert method on SQLiteDatabase returns the row ID of the newly inserted row (or -1 if an error occurred), Model.save might as well pass this along instead of discarding it. It can be useful when you create a transient model, persist it and then need to know the value the database automatically assigned to Id when it was inserted.

See [SQLiteDatabase docs](http://developer.android.com/reference/android/database/sqlite/SQLiteDatabase.html#insert(java.lang.String, java.lang.String, android.content.ContentValues)).