Update camera.js

1.a Noticed "stop script" alert in FireFox-Linux environment, since the Camera.js is executing the slideshow non-stop.
1.b The Slideshow kept running, independent of the audience i.e even in the background.
   . Used 'PageVisibility' API to pause the Slideshow.
   . Resume the Camera slideshow accordingly, when the Page becomes visible.
   . Mobile browsers support the PageVisibility API.
   . < IE10 browser does not support the PageVisibility API (http://caniuse.com/#feat=pagevisibility)
2. Noticed the app does not support FireFox OS.
   . Added the 'Mobile' search term to identify a Mobile device inside the Navigator.UserAgent. 
   . FireFox OS will be having 'Mobile' in the UserAgent.