Code review: Speculative implementation of the Resource Timing API

So this is the (rough) code I have for https://github.com/lognormal/boomerang/issues/25. I haven't tested it in all the browsers yet, just wanted to know whether I'm on the right track, especially with regards to adding a `post_url` option. Let me know what you think.

(also, apologies if this was the wrong repo to open the pull request against, I wasn't sure which was correct)