Added Haxe 3 support

I amended imports of js.DOM to

#if haxe3
import js.html.HtmlElement;
#else
import js.Dom;
#end 

And one reference to HtmlDom will be changed to HtmlElement in haxe 3

The test runs correctly in haxe 2.10 and haxe 3.0.0

Cheers,
Nick H