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 branch 'ff-mutationobserver' into observers

Conflicts:
	lib/reddit_enhancement_suite.user.js