Request termination for XHR

This implements abort and timeout for XHR.

[Here's the wpt run result](http://manishearth.anapnea.net/xhr/logtm3.html). Ignore the timeouts, they're due to a bug in wpt (Need to [upgrade wpt](https://github.com/mozilla-servo/web-platform-tests/pull/1) to fix it). The two failures for abort tests have been fixed.

Blocks #2282