Use externs from lib/accessibility-developer-tools and use content script context for checking correct frame URI in sidebar.js
Modify AuditRule to have a relevantElementMatcher method which returns true/false rather than a relevantNodeSelector method which returns a collection of elements. AuditRule now walks the DOM tree by hand.
Merge pull request #4 from alice/master

Update to use relevantElementMatcher and use externs from library.