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

Also fixed a bug in addGroup(): SELECT COUNT (NULL) doesn't work.

Offered and requested message IDs are stored in the database rather than
being owned by DuplexConnections. This paves the way for moving to a
simplex transport layer.

The distinction between inbox groups and other groups can be maintained
internally, there's no need to represent it on the wire.

Private messages are now the same as group messages, but groups can be
private or public. When a contact is added, a private group is created
and designated as the inbox for exchanging private messages with the
contact.

Removed Commons IO, which we were only using as a replacement for
File.getFreeSpace() on desktop plaftorms.

Note: The Huawei U8210 (Android 2.1) doesn't have all the Java 1.6
standard library methods, and crashes if they're called. Specifically,
String.isEmpty() and NetworkInterface.supportsMulticast() are missing,
so the changes removing those methods were not reverted.

The Request packet now contains a list of message IDs, rather than a
bitmap referring to the list of messages IDs in the Offer. This allows
the Request to be understood out of context, e.g. if the Offer and
Request are sent over separate connections or a connection is replayed.

If the group's owner is spamming, unsubscribe.

(Although it isn't possible to create an identity yet...)

The invitation and private messaging UIs are currently broken. Some key
rotation bugs were fixed; others may have been created (unit tests
needed). An encoding for private keys was added. Pseudonyms were moved
out of the messaging package and ratings were moved in.

Fields in Android UI objects that are accessed from background threads
must be declared volatile. UI objects use data attached to DB events to
avoid DB lookups, which complicates the UI code but should improve
performance.