Proposed changes to BPModel class.

Fixes #1 
* The existing NSAssert calls had no function (due to first parameter being always YES).
* We thought it better not to disallow/warn about calling of methods in the super class that are not necessarily meant to be overridden in subclasses.
* Both serializer and transformers getter methods will now return empty dictionaries to promote the habit of always "building on top of" these properties when overriding in subclasses. The writer of a subclass should not have to worry about getting nil instead of an NSDictionary instance from these getters.