[preload, resource-hints] spec updates

Been working my way through various open issues on Preload and Resource
Hints. FYI:

-
https://github.com/w3c/resource-hints/compare/gh-pages@%7B2015-03-30%7D...gh-pages@%7B2015-04-05%7D
-
https://github.com/w3c/preload/compare/gh-pages@%7B2015-03-30%7D...gh-pages@%7B2015-04-05%7D

Also, started work on a larger refactor across both specs to remove
"loadpolicy" attribute in favor of using rel=prefetch and rel=prerender -
see [1] for context. Previously the latter two were defined via "next" and
"inert" keywords on loadpolicy:

- https://github.com/w3c/preload/pull/17
- https://github.com/w3c/resource-hints/pull/27

ig

[1] https://github.com/w3c/resource-hints/issues/26