feat(ngModelOptions): custom triggers and debounce of ngModel updates

By default, any change to an input will trigger an immediate model update,
form validation and run a $digest. This is not always desirable, especially
when you have a large number of bindings to update.

This PR implements a new directive `ngModelOptions`, which allow you to
override this default behavior in several ways. It is implemented as an
attribute, to which you pass an Angular expression, which evaluates to an
**options** object.

All inputs, using ngModel, will search for this directive in their ancestors
and use it if found.  This makes it easy to provide options for a whole
form or even the whole page, as well as specifying exceptions for
individual inputs.

* You can specify what events trigger an update to the model by providing
  an `updateOn` property on the **options** object. This property takes a
  string containing a space separated list of events.

  For example, `ng-model-options="{ updateOn: 'blur' }"` will update the
  model only after the input loses focus.

  There is a special pseudo-event, called "default", which maps to the
  default event used by the input box normally. This is useful if you
  want to keep the default behavior and just add new events.

* You can specify a debounce delay, how long to wait after the last triggering
  event before updating the model, by providing a `debounce` property on
  the **options** object.

  This property can be a simple number, the
  debounce delay for all events. For example,
  `ng-model-options="{ debounce: 500 }" will ensure the model is updated
  only when there has been a period 500ms since the last triggering event.

  The property can also be an object, where the keys map to events and
  the values are a corresponding debounce delay for that event.
  This can be useful to force immediate updates on some specific
  circumstances (like blur events). For example,
  `ng-model-options="{ updateOn: 'default blur', debounce: { default: 500, blur: 0} }"`

This commit also brings to an end one of the longest running Pull Requests
in the history of AngularJS (#2129)!  A testament to the patience of @lrlopez.

Closes #1285, #2129, #6945