Refactor _ReverseReferenceProperty.__get__

Move model type specific code to a get_reference_property method in
botoweb.db.coremodel.Model and botoweb.db.dynamo.DynamoModel.
Add a SetProperty subclass of botoweb.db.property.Property
Style changes.
Merge pull request #5 from debroys/feature/tower-compatibility

Feature/tower compatibility