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.

Broken example (use firefox):
http://plnkr.co/edit/jDbt7gJqxe0Y4G5hnogu?p=preview

Working example (use firefox)
http://plnkr.co/edit/06IKn2WWq0ttvhAdf2Iq?p=preview

Closes #3053