Use testling to run tests

[RickEyre]

If we want to make a real shim we need to be able to test on all browsers. We can do this a number of ways.

We can set up our own testling server and then have a git commit hook, or GitHub pull request hook, to run our tests. Or we can set up a local testling and Travis can run it for us hopefully.

If we get this set up we can separate the NodeVTT stuff out into a separate repo and pull it in for cue2json. Or even have NodeVTT do the stuff that cue2json does.

[RickEyre]

After #196 lands and it's landed in Gecko I think this should be the next priority as we're getting some issues on PhantomJS such as some CSS properties like unicode-bidirection and CSS units like 'vh' and 'vw' not being supported. document.createProcessingInstruction doesn't work as well. I'm in the middle of trying to figure out how to get PhantomJS to support these things (as they should since it's in there docs), but I think we might have to compile a version of PhantomJS ourselves to get that to work. Instead of going through all this we should just switch to Testling and switch node-vtt out into it's own repo.