Enable link hints across all frames of a tab

This PR enables link hints in all frames of a page simultaneously, managed by a "link hint oracle" in the background page. It fixes #602.

8/32 DOM tests now fail, haven't looked into fixing them yet.

Some examples to test it out on:
* [This frameset demo](http://www.quackit.com/html/templates/frames/frames_example_2.html).
* [/r/videos](http://www.reddit.com/r/videos), clicking on any of the 
![screenshot from 2014-10-24 06 42 16](https://cloud.githubusercontent.com/assets/1847343/4765582/9fec5f10-5b40-11e4-964b-c96175bdca01.png)
buttons to open an iframe, and then using link hints seamlessly between the page and the iframes.
* The w3schools ["iframe try-it-out"](http://www.w3schools.com/tags/tryit.asp?filename=tryhtml_iframe) (you'll probably want to `gf` to the iframe and scroll it down a bit so the links are visible first).