Sets z-index for .offline-ui to a higher value

Sets z-index to maximum allowed value
(http://softwareas.com/whats-the-maximum-z-index) in order to ensure the
UI elements always remain above any other elements rendered to the page.
Changed z-index to 2000
Merge pull request #68 from craigshoemaker/master

Sets z-index for .offline-ui to the maximum allowed value