Adds haxe.Http.cancel for js and flash9

Adds an instance method to `haxe.Http` to allow you to cancel an active request before it has returned. This functionality was needed in one of our libraries, and as there was no reference maintained to the XHR/URLLoader we had to duplicate the STD class.

I'm open to suggestions on the method name, we had called it `cancel` in `mloader` so I stuck with that. In JS it is `abort` and in AS3 it is `close`.