Set up continuous integration

[RickEyre]

Using something like https://ci.testling.com/. That way on every pull request we can get pass/fail information on various browsers.

[inexorabletash]

We should strip down the UTF-8 tests; right now they try and test the entire Unicode scalar value space, which is quite slow.

I think someone did a test variant that examined only a subset. I'll try and dig that up.

[inexorabletash]

I'm still open to this idea. One tricky but is that since it's a polyfill the native implementation will be tested on browsers that support it, rather than the polyfill itself.