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.

Test page: http://sharemenot.cs.washington.edu/Test%20Installation.shtml.

(To test, manually block all trackers on the page via Privacy Badger's UI. Might need to explicitly get cookies for some to show up, such as by visiting facebook.com or pinterest.com before testing.)

There's currently no user-facing way to disable button replacement, but the socialWidgetReplacementEnabled pref controls it.
Is this fixed by specifying `true` as the third argument to `button.addEventListener` in order to prevent the event from bubbling up the DOM tree?