Added marking queue, and made tutor clients callbacks.

Minor changes.

Implemented persistent tutor client. ...

Hopefully this solves the weird issues we were seeing with the tutor
clients deadlocking.  Will also make the Android client simpler which
should again reduce race conditions.

Code cleanup. Been playing with a few different static code analysis ...

tools.  Used them to recommend the changes made in this commit.

Clean up. Fixed logging statements, added private constructors to ...

classes that should never be instantiated.  Fixed author names to be
consistent.

Cleanup, and further improvements to concurrency. ...

Should hopefully be bullet proof now.  Seems to handle the stress test
OK.  The tutor client doesn't lose sync during stress test, so hopefully
that problem is now fixed.

Changed to mousePressed rather than mouseClicked because clicking labels is a ...

bit weird (seem less tolerant of movement during click than buttons).

Also attempting to fix problem with tutor client occasionally losing sync which
is probably due to threading (maybe server request thread deadlocking).

Improved TutorClient. Huge refactor. Icon now read from classpath/jar.

Reworked threads, shutdown hook, and exit process to ensure requests are cancelled if student logs/out without cancelling manually.

Done a whole bunch of stuff: ...

- Added service discovery.
 - Refactored the IMapPanel interface and LabelProcessor to make panels simpler.
 - Numbers now end up in queue label.

Nearly finished.  Need to do testing in labs.

Initial commit.