Changes for uPortalUP-3785 Implement Background Changer portlet on mobile web (https://issues.jasig.org/browse/UP-3785)

Implemented the background changer portlet into the umopile-ui code.
Changed the code based on Pull Request comments from Matt
Comments cleanup

Changed the executeBackgroundScript to !_.isDefined(…) in resource/Background.js
Removed @return from comment block for the toggleVisibility method
Merge pull request #41 from mindblender/master

uPortalUP-3785 Implement Background Changer portlet on mobil...
Merge upstream/master