[resource-hints] draft update, merged processing model (preload+prerender)

*tl;dr:* preload and prerender have high functional overlap and subtle
differences in their processing policies -- unnecessary API complexity,
hard to explain, not as flexible. To address this, latest draft merges
prerender hint into preload, and introduces "loadpolicy" concept that
allows you to control fetch and execution policies for each fetch.
Functionally, the capabilities are the same, this is an API refactoring...

- added loadpolicy with "inert" + "next" keywords
- updated processing model to reflect new "loadpolicy" attribute
- renamed "probability" atribute to "pr"

Latest draft: w3c.github.io/resource-hints
Relevant discussion: https://github.com/w3c/resource-hints/issues/4

Would appreciate any feedback or comments!

ig

P.S. new API also aligns with the proposed resource loading discussion on
whatwg:
http://lists.w3.org/Archives/Public/public-whatwg-archive/2014Oct/0104.html