[resource-hints] draft updates, now in w3c/web-performance repo

I've moved the draft spec into the W3C webperf repo. Latest version:
https://w3c.github.io/web-performance/specs/ResourceHints/Overview.html

A few highlights since last update:
- "context" attribute -> "as" [1]
- "probability" attribute to indicate speculative fetches [2]
- documented emitted events [3]
- other small terminology improvements

Also, if you're not following the whatwg list, Ian has a proposal out for
"Preloading and deferred loading of scripts and other resources" [4], which
has some overlap with rel=preload. No conclusions yet, but a good
discussion.

ig

[1] https://github.com/igrigorik/resource-hints/issues/21
[2] https://github.com/igrigorik/resource-hints/issues/17
[3] https://github.com/igrigorik/resource-hints/issues/4
[4]
http://lists.w3.org/Archives/Public/public-whatwg-archive/2014Sep/0012.html