refactor(ngModelOptions): move debounce and updateOn logic into NgModelController

Move responsibility for pending and debouncing model updates into `NgModelController`.
Now input directives are only responsible for capturing changes to the input element's
value and then calling `$setViewValue` with the new value.

Calls to `$setViewValue(value)` change the `$viewValue` property but these changes are
not committed to the `$modelValue` until an `updateOn` trigger occurs (and any related
`debounce` has resolved).

The `$$lastCommittedViewValue` is now stored when `$setViewValue(value)` updates
the `$viewValue`, which allows the view to be "reset" by calling `$rollbackViewValue()`.

The new `$commitViewValue()` method allows developers to force the `$viewValue` to be
committed through to the `$modelValue` immediately, ignoring `updateOn` triggers and
`debounce` delays.

BREAKING CHANGE:

This commit changes the API on `NgModelController`, both semantically and
in terms of adding and renaming methods.

* `$setViewValue(value)` -
This method still changes the `$viewValue` but does not immediately commit this
change through to the `$modelValue` as it did previously.
Now the value is committed only when a trigger specified in an associated
`ngModelOptions` directive occurs. If `ngModelOptions` also has a `debounce` delay
specified for the trigger then the change will also be debounced before being
committed.
In most cases this should not have a significant impact on how `NgModelController`
is used: If `updateOn` includes `default` then `$setViewValue` will trigger
a (potentially debounced) commit immediately.
* `$cancelUpdate()` - is renamed to `$rollbackViewValue()` and has the same meaning,
which is to revert the current `$viewValue` back to the `$lastCommittedViewValue`,
to cancel any pending debounced updates and to re-render the input.

To migrate code that used `$cancelUpdate()` follow the example below:

Before:

```
  $scope.resetWithCancel = function (e) {
    if (e.keyCode == 27) {
      $scope.myForm.myInput1.$cancelUpdate();
      $scope.myValue = '';
    }
  };
```

After:

```
  $scope.resetWithCancel = function (e) {
    if (e.keyCode == 27) {
      $scope.myForm.myInput1.$rollbackViewValue();
      $scope.myValue = '';
    }
  }
```