TouchEvents pull request: add TouchEvent constructor

I added this pull request <https://github.com/w3c/touch-events/pull/16> a
few weeks ago but apparently there was no notification to
public-touchevents :-(.  Please take a look.  If there are no objections in
the next few days I'll merge it in.

My main motivation for doing this now is the InputDevice work
<https://github.com/rbyers/InputDevice>.  Without a constructor, any new
properties of UIEvent technically should include an extention to
initTouchEvent, which I really don't want to do.  As with initTouchEvent, I
don't expect a lot of use here, but we should have a constructor for
completeness.

If this group agrees to the spec change, I expect we'll add it to blink
shortly (and given the low usage, we may even be able to deprecate
initTouchEvent etc. before long).

Thanks,
   Rick