fix($animate): ensure that a timeStamp is created if not provided by the browser event

Firefox and (sometimes) Opera may not provide a timeStamp value in their event when passed
to the event handler. This may cause animations not to close properly. This fix will automatically
create a timeStamp value for the event in this situation when missing.

Closes #3053
Merge branch 'master' of https://github.com/angular/angular.js
componentizd!