Notify User if the talk page exists

Upon typing a title, the plugin will check for the main page, the talk
page, a draft page in the format 'draft:title' and the talk page for the
draft, in that order of importance

Known issues: The error message flashes a little when the name is being
typed, due to the different namespaces being checked.

http://www.google-melange.com/gci/task/view/google/gci2014/5877487489777664
https://github.com/WPAFC/afch-rewrite/issues/26
It's not necessary to check if a draft page exists -- because this page itself IS a draft page! (That is, the user is already reviewing the draft page... so it would make sense for it to already exist :) ).