By default, any change on the content will trigger an immediate model update
and form validation. This PR implements a new directive `ng-model-options`
that allow you to override this default behavior in several ways.

You should specify an object with the different parameters.

For example, it allows to trigger an update only when a particular event or
list of events is received by the input using the `updateOn` key. Should you
need multiple events, just assign an array to it.

I.e. `ng-model-options="{ updateOn: 'blur' }"` will update and validate only
after the control loses focus.

If you want to keep the default behavior and just add new events that may
trigger the model update and validation, add "default" as one of the specified
events.

I.e. `ng-model-options="{ updateOn: ['default','submit'] }"`

Also, with the `debounce` option, `ng-model-options` will allow deferring the
actual model update until a timer expires. The timer will be reset each time
an event is triggered.

I.e. `ng-model-options="{ debounce: 500 }" for 500ms after the latest event.

Custom timeouts for each event can be set for each event if you use an object
in `debounce`. This can be useful to force immediate updates on some specific
circumstances (like blur events).

I.e. `ng-model-options="{ updateOn: ['default', 'blur'], debounce: { default: 500, blur: 0} }"`

You can use the directive in any tag so its contents became the default settings
for any child control, although they can be overridden.

Closes angular#1285, angular#2129