Support for nested models and collections

[mikeapr4]

• Deep nested enhanced proxy interface for model/collection hierarchy
• Nested data made reactive for a Vue instance up to recursion. This means that A->B->A won't be reactive within a single component, but that if A->B happens in one component, and a child of that component does B->A, that will be reactive. Very unlikely that someone would come across this scenario really, but it does exist.
• Top-level model relational changes are reactive.

Work Remaining:

• Relational collection changes aren't reactive, this is because only the top-level model/collection is bound to a Vue component.

[coveralls]

Coverage decreased (-14.0%) to 86.034% when pulling c11a5ed on enhancement/backbone-associations into 4eaaaaa on master.