Added reactive.get(fn) & reactive.set(fn) adapters and updated internals

This PR does a few things:

1. Adds the remaining adapters which is important for interfacing with other libraries. 
2. The library now uses these getters and setters consistently internally, instead of hardcoded `obj[prop]` and `obj[prop]()`.
3. Added backbone example to show how these adapters will work
4. Finished adapter tests
5. Removed binding-specific custom subscribe and unsubscribe fns. They weren't being used anywhere and there were no tests for them. Couldn't think of a use case where this would be needed, but if there was a reason, I can add them back in.

All tests pass with no modifications to original tests, so it shouldn't break anything.

This PR also will make it really easy to create two-way binding plugins as the view now knows how to update the model.