gnorb-utils.el: Require mailcap, and parse

This could be a bad idea, but it might be necessary for attaching
exported files to outgoing messages.
gnorb-org.el: Email current subtree as exported text or file

Basic works-for-me functionality, bugs to be discovered later. About
four minutes after I wrote this I discovered there's some overlap with
org-mime, but it's different enough to keep.
README.org: Document new capture attachment thingy.
README.org: Document gnorb-org-email-subtree