feat(jqLite): implement the `empty` method

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.

An example benchmark: http://jsperf.com/html-vs-empty shows extreme performance gains.

It would be great to have it for 1.2.0, especially that it doesn't introduce any breaking changes.