Include document data in the dump.

Include document data in the dump.

At the same time, I've started hacking up sup with a new NotmuchIndex
class in the place of the previous XapianIndex class. The new class
stores only the source_info field in the document data, (rather than
a serialized ruby hash with a bunch of data that can be found in the
original message).

Eventually, I plan to replace source_info with a relative filename for
the message, (or even a list of filenames for when multiple messages
in the database share a common message ID).