Add full-text indexing using the GMime library for parsing.

This is based on the old notmuch-index-message.cc from early in
the history of notmuch, but considerably cleaned up now that
we have some experience with Xapian and know just what we want
to index, (rather than just blindly trying to index exactly
what sup does).

This does slow down notmuch_database_add_message a *lot*, but I've
got some ideas for getting some time back.