Make all bindings hold reference to the actual key at which the model resides on the view.

This allows us to update the bindings' model property according to the
models currently set on the view.
Add update functions on both Rivets.View and Rivets.Binding used for updating bindings with a new set of models. [Closes #131]
Merge pull request #154 from mikeric/view-context-updating

View context updating