perf(jqLite): implement and use the empty method in place of html(‘’)

jQuery's elem.html('') is way slower than elem.empty(). As clearing
element contents happens quite often in certain scenarios, switching
to using .empty() provides a significant performance boost when using
Angular with jQuery.

Closes #4457

test/ngAnimate/animateSpec.js

@@ -301,7 +301,7 @@ describe("ngAnimate", function() {
           inject(function($animate, $compile, $rootScope, $timeout, $sniffer) {
 
           $rootScope.$digest();
-          element.html('');
+          element.empty();
 
           var child1 = $compile('<div>1</div>')($rootScope);
           var child2 = $compile('<div>2</div>')($rootScope);