Re: [touch-events] Add a constructor for TouchEvent and Touch (#16)

A couple areas to pay attention to: 

The EventTarget isn't supposed to be nullable, so we can't have a default value - so I made it required (along with the `identifier` for good measure).  But note that in blink at least, it is actually nullable - `document.createTouch()` will create a `Touch` object with a `null` target).

Like for `createTouch` I didn't allow `clientX` and `clientY` to be explicitly initialized.  Instead they're initialized implicitly by `pageX` and `pageY`.  This is similar to the `MouseEvent` constructor, except there it's the client co-ordinates that are explicit with the page ones computed from them.  Perhaps it's better to follow the `MouseEvent` pattern?

I initializes the `TouchList` objects with a `sequence<Touch>` (not unlike [the FontFaceSetLoadEvent constructor](http://dev.w3.org/csswg/css-font-loading/#dictdef-fontfacesetloadeventinit)).

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