2 new rules, removing console spam, minor tweaks, new settings page

New in this version:

• Two new audit rules ("An element's ID must not be present in more that one aria-owns attribute at any time" and "Avoid positive integer values for tabIndex")
• Generalized "aria-labelledby attributes should refer to an element which exists in the DOM" to "ARIA attributes which refer to other elements by ID should refer to elements which exist in the DOM"
• Improved logic for “The purpose of each link should be clear from the link text”
• New-style settings page available in Chrome M40 and higher
• Fixed annoying "axs is not defined" spam in console.

Minor bugfix release

Fixes issue with sidebar not working on pages/frames with URIs which have fragments.

Support Shadow DOM

Other improvements:

• Limit number of results per audit rule to 100
• Add 'Focus' properties in sidebar pane

v2.8.0: Merge pull request #5 from alice/master

Request extensions prefs via the background page