feat($http): implement mechanism for coalescing calls to $apply in $http

When multiple responses are received within a short window from each other, it can be wasteful to
perform full dirty-checking cycles for each individual response. In order to prevent this, it is
now possible to coalesce calls to $apply for responses which occur close together.

This behaviour is opt-in, and the default is disabled, in order to avoid breaking tests or
applications.

In order to activate coalesced apply in tests or in an application, simply perform the following
steps during configuration.

   angular.module('myFancyApp', []).
     config(function($httpProvider) {
       $httpProvider.coalescedApply(true);
     });

OR:

   angular.mock.module(function($httpProvider) {
     $httpProvider.coalescedApply(true);
   });