Store models reference on view

This PR introduces a reference on every view that points to the original models object that it is scoped to. 

Currently, in the 'each-*' binding routine, the models object gets rebuilt for independence among the sub-views. This means that when you are inside of an event callback, you cannot alter properties on the original object.

Adding a reference to the original model, allows an event binding handler like this:
```javascript
  rivets.configure({
    handler: function(context, ev, binding) {
      this.call(binding.view.modelsRef, ev, binding.view.models);
    }
  });
```
And this terribly contrived, but hopefully demonstrative, view:
```javascript
  function View() {
    this.data = 'foo';
    this.count = 0;
  }

  // function bound within an iteration block
  View.prototyp.myClickEvent = function(e, ctx) {
    this.data = ctx.someIteratedModel;
    this.count++;
  }
  
  rivets.bind(div, new View)