Move MutationObserver into BrowserDetect out of global scope

var MutationObserver = window.MutationObserver doesn't work in FF19 because scoping.  Therefore, let's avoid the scoping issue by storing RES's pointer to MutationObserver in an object (BrowserDetect).
Merge pull request #275 from andytuba/ff-mutationobserver

Move MutationObserver into BrowserDetect out of global scope