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.

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.

This should make it possible to develop the core in Eclipse without the ADT.

Retrieve messages from the database in raw form to avoid creating unnecessary short-lived objects. Added timestamps to headers.

Rewrote the bundle reading and writing code to eliminate copying. Signatures and digests are now calculated on the fly as the data is read or written. This is a little bit tricky in the case of reading because ReaderImpl uses a lookahead byte, so the signature and message digest need to lag one byte behind.