Use prototype inheritance in each-binder

This fixes a problem where arrays and objects where not updated inside the each-binding.

I've created a JSFiddle to demonstrate the problem: http://jsfiddle.net/hnodsc7n/1/
When the `items`-array is changed, the each-binding still holds a reference to the old `items`-array. This is because rivets just copies the properties from one model to another. However, this fix uses prototype inheritance. Thus every change on the super-model is propagated to the sub-model. Check out the difference: http://jsfiddle.net/znv28cmj/1/

There are two potential problems with this fix:

- I'm using `Object.create()` which is not available in IE8. However, there is a polyfill for it (which I haven't included because imho libraries shouldn't bring their own polyfills).
- If someone is iterating over the sub-model and checks for `if (obj.hasOwnProperty(key))` it will not iterate over all properties anymore.

All tests are running