Support for 'animationstart' event

### See #35 for the original idea.

Maybe rename `animationstart` to `animate`, as we don't have an `animationend` right now.

**TODO:**

- [x] add tests before merging
- [x] should use [``"CustomEvent"`][1] as eventType on IE9+
- [x] should use [Event Constructor][2] in new browsers

[1]: http://msdn.microsoft.com/en-us/library/ie/ff975304(v=vs.85).aspx
[2]: https://developer.mozilla.org/en-US/docs/Web/API/CustomEvent