Use `new MouseEvent()` when possible

[scottgonzalez]

See http://trac.webkit.org/changeset/140657

[NiGhTTraX]

I tried implementing this and the QUnit tests fail due to a bug in PhantomJS. It turns out that it doesn't treat the new event constructors as actual constructors. See ariya/phantomjs#11289 for details.

They marked it for milestone 2.0, so once that comes out I'll take another shot at it.

[jzaefferer]

Looks like PhantomJS 2.0 will take a least several more months. SlimerJS isn't much of an option either, since it doesn't run headless on Linux - we need that for Jenkins and Travis. And I really don't want to mess with xvfb again.