fix($sanitize): consider `size` attribute as valid/allowed attribute

The "size" attribute gets set on <font> elements when using HTML5 rich
text editors, or elements with the contenteditable attribute, that rely
on the 'fontSize' command (execCommand).

Closes #5522
fix(ngRoute): instantiate controller when template is empty

Before this change, $route controllers are not instantiated if the template is falsy, which includes
the empty string. This change tests if the template is not undefined, rather than just falsy, in
order to ensure that templates are instantiated even when the template is empty, which people may
have some reason to do.

This "bug" was reported in http://robb.weblaws.org/2013/06/21/angularjs-vs-emberjs/, as a "gotcha"
for AngularJS / ngRoute.

Closes #5550
perf(Scope): limit propagation of $broadcast to scopes that have listeners for the event

Update $on and $destroy to maintain a count of event keys registered for each scope and its children.
$broadcast will not descend past a node that has a count of 0/undefined for the $broadcasted event key.

Closes #5341
Closes #5371