Fixes #360. Adds support for deep subModelType hierarchies.

Recursively resolves the appropriate subModelType to instantiate in
'Backbone.RelationalModel.build'. This allows subModelTypes to in turn declare
their own subModelTypes.
Need to initialize the superModel's heirarchy before initializing the current model's hierarchy.
Pulling the relation-inheritance logic out of Backbone.RelationalModel.initializeModelHierarchy into it's own method so that initializing super-class relations doesn't cause the super-class's model hierarchy to become initialized. There are situations where initializing the super-class's model hierarchy during relation inheritance will prevent some relations from being inherited. Updated the logic in Backbone.RelationalModel.initializeModelHierarchy that recursively initializes the model hierarchy of subModels so that it doesn't initialize models that have already been initialized.
Merge pull request #362 from kartofflly/deep-submodeltype-hierarchies

Fixes #360. Adds support for deep subModelType hierarchies.