notmuch-index-message: Add code to actually create a Xapian index

notmuch-index-message: Add code to actually create a Xapian index

Most of this code is fairly clean and works well. One part is
fairly painful---namely extracting the body of an email message
from libgmime. Currently, I'm just extracting the offset to
the end of the headers, and then separately opening the message.
Surely there's a better way.

Anyway, with that the results are looking very similar to sup-sync
now, (as verified by xapian-dump). The only substantial difference
I'm seeing now is that sup does not seem to index quoted portions
of messages nor signatures. I'm not actually sure whether I want
to follow sup's lead in that or not.