feat(form):  add methods to set ng-working class, ng-idle class, $workin...

Forms submissions often require interaction with the server.  Other items
on the form, as well as parent forms, should be able to know if
outstanding interactions are in process so buttons can be disabled or
changed to a working state, etc.  This feature adds native support in the
angular form so that form elements can change the global state of the form
based on whether or not these interactions are pending.