Associate outgoing messages with arbitrary headings

Previously, calling gnorb-gnus-outgoing-do-todo on messages under
composition would automatically either make a new TODO, or trigger state
change on an existing todo. With a prefix arg, now allow the user to
choose an arbitrary heading that this message should be associated with.

lisp/gnorb-gnus.el: (gnorb-gnus-outgoing-do-todo) prefix arg prompts for
		    new heading to associate message with.
Wishlist changes

README.org: handle-mail and email-subtree are now merged, make note of
	    automatic org tagging for BBDB contacts.
Clearing list of saved attachments

* lisp/gnorb-utils.el (gnorb-trigger-todo-action): This is where
  gnorb-gnus-capture-attachments should be set to nil.

* lisp/gnorb-gnus.el (gnorb-gnus-incoming-do-todo): Don't do it here.
Use = not eq for numbers

* lisp/gnorb-utils.el (gnorb-scan-links): Don't use 'eq for numbers
Don't use `read' when there's nothing to read

* lisp/gnorb-gnus.el (gnorb-gnus-outgoing-do-todo): If read is fed nil,
  it prompts for a lisp expression! Make sure it's not fed nil.
Improvements to TODOs made from outgoing messages

* lisp/gnorb-gnus.el (gnorb-gnus-check-outgoing-headers): We don't need
  to store quite so much information, because we're using more existing
  extraction functions later on.
  (gnorb-gnus-outgoing-make-todo-1): Fix basically broken capturing when
  a real link is available. Add more escapes when it isn't. Use
  existing functions `org-add-link-props' and `org-store-link-props' to
  integrate a little better.

Continued thanks to Alan Schmitt, Gnorb's first and possibly only user.