Very important stuff.

Bug #47.

Of course, not all devices support Bluetooth...

This allows the oldest LocalAuthor to be used as the default.

All JVMs must support UTF-8 encoding.

This adds complexity but will save a lot of bandwidth, as most of the
strings and byte arrays we want to send are less than 128 bytes.

The extra complexity isn't exposed outside of the serial component.

We check for compatibility by specifying a schema version and minimum
schema version in the code, storing them in the DB, and checking whether
the DB is too old for the code or vice versa.

Dev task #52. Known but unverified identities are also supported, but
currently unused. These will be used in future for contacts who've been
introduced but not verified face to face.

This should've been changed when restricted groups were removed.

The new format is simpler but less efficient for small integers, short
strings and short byte arrays.