README.org: Reorganize README

Reorganize the file, and make suggested keybindings easier to find.
README.org: Add keybindings to gnus mime commands.

It's ugly (involves copying and re-running the code that sets
gnus-mime-button-map), but it works.
gnorb-org.el: new option gnorb-org-mail-todos

gnorb-org-restore-after-send checks for an appropriate TODO keyword
before calling org-agenda-todo.
gnorb-utils.el: gnorb-prompt-for-bbdb-record

Change wording of prompt.
gnorb-org.el: gnorb-org-handle-mail, gnorb-org-email-subtree

Refactoring of these two functions, they now share more functionality.
Not actually sure that's a good thing -- going to use them for a while
and see.