Use actual browser tests instead of Jest

[cafreeman]

Since gjalljarhorn is ultimately abstracting browser APIs, it'd be ideal to be able to run tests in the browser and actually use that API. As it stands, I don't really have a way to test the actual timing portion of the library since I have have to mock window.performance. Instead, we can only test the behavior of the Timer class itself.

The other option would be to make a more feature-complete performance mock that does its own timing (maybe with process.hrtime?), but I'm not sure which is preferable.