Same exponential backoff approach as Bluetooth.

DuplexOutgoingSession flushes its output stream if it's idle for a
transport-defined interval, causing an empty frame to be sent. The TCP
and Tor plugins use a socket timeout equal to twice the idle interval to
detect dead connections.

See bugs #27, #46 and #60.

We don't need two separate executors for long-running IO threads.

Of course, not all devices support Bluetooth...

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 allows different plugins to have different dependencies without
cluttering the plugin factory API.

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

The project is now built as an Android project (via Eclipse or
ant). Tests have been moved to a separate project so they can exist
outside the Android build process. A basic Android app structure has
been created. A Bluetooth plugin for Android has been added, and the
Bluetooth plugin for J2SE has been modified to use the same techniques.

Unidirectional transports and connections are now called
simplex rather than batch. Bidirectional transports and connections
are now called duplex rather than stream.

No Thread.sleep() till Brooklyn.