fix(tab): IE9 disabled attr renders grey text on enabled tab

This issue can be seen on the boostrap UI page using IE9.  The second tab looks disabled when it is really enabled:
http://angular-ui.github.io/bootstrap/#/tabs

- IE9 renders elements with 'disabled' property with grey text
- Tabs are rendered with grey text even if they are not disabled
- By renaming attribute to 'disable' instead of 'disabled' tabs with dynamic 'disable' attribute still render correctly