Skip Note items when scanning state changes

lisp/gnorb-org.el: gnorb-org-scan-state-notes will now skip any leading
		   - Note items in the logbook, and start checking with
		   the first - State item.

This is a temporary measure. If gnorb-org-mail-scan-state-changes is set
to 'all, this will only skip the leading - Note items, not those later
in the drawer. But really, who would want to scan all the state-change
notes? This needs to be much more flexible and customizable, so people
can make gnorb-org-handle-mail do just what they want.
Remember to put :group and :type on defcustoms

lisp/gnorb.org.el: Forgot gnorb-org-mail-scan-function,
		   gnorb-org-msg-id-key, and gnorb-org-mail-todos
New option gnorb-org-find-candidates-match

lisp/gnorb-org.el: When gnorb-org-find-visit-candidates looks for org
		   headings relevant to the current message, this string
		   can be used as a org-tags-matcher to limit which
		   headings are visited.
Don't prompt for action if todo is not possible

lisp/gnorb-utils.el: gnorb-trigger-todo-action shouldn't prompt you for
		     an action if you've already screened out the
		     possibility of a todo change state.