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.

This produces a saner user experience when devices have differing
clocks.

Older devices such as the Huawei U8210 don't completely support the Java
1.6 API. Calling an unsupported method crashes the app.

Older devices such as the Huawei U8210 don't completely support the Java
1.6 API. Calling an unsupported method crashes the app.

This allows quick access to any part of the app, including quitting (two
touches).

This makes the contact list behave how alpha testers expected: touching
a contact shows private messages exchanged with the contact.

As we no longer support Java 1.5 we can stay up to date with H2
releases.

Four more warnings were left unfixed because quick fixes would've broken
the tests.

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.