Use jQuery UI styles from Sifter

Fixes issue #9

- Use the "mentions" autocomplete style
- Suppress the jQuey UI accessability message
- Remove the jQuery UI CSS and image as we use Sifters CSS and images now
- Strip CSS and images from Chrome web_accessible_resources
- Modified jQuery UI JS so that it applies the "ui-state-hover" on focussing/hovering over an element instead of "ui-state-focus" so Sifter styles will apply correctly.