Make the timeago directive expand into a standard HTML5 time element

This PR makes the `amTimeAgo` directive to always expand into a `<time>` element per HTML5 standards:

https://developer.mozilla.org/en-US/docs/Web/HTML/Element/time
http://html5doctor.com/the-time-element/

While there are 6 commits, the one that matters is 1c24b2660f99da3db0ab1d93b884fa50959423b7.