Integrate ShareMeNot functionality

This branch supports ShareMeNot's social widget replacement functionality. Buttons are replaced if the corresponding domains are blocked by Privacy Badger, and those domains are temporarily unblocked if the button is clicked.

This functionality is enabled by default (Utils.isSocialWidgetReplacementEnabled() just returns true), but should be linked to a user-facing setting.

Test page: http://sharemenot.cs.washington.edu/Test%20Installation.shtml. 

Caveat: Button replacement messes with webpage layout on some pages, so this should likely be an opt-in feature.