Make test_mantis_server loop after dealing with a request.
Added abstract member cancel() to callable and implemented empty version in all derived callable classes.  callable::cancel is called when thread::cancel executes.
Added signal_handler and digitizer_test
Some cleanup in mantis_server
Added use of signal_handler to mantis_server.
Fixes in signal_handler and digitizer_test.
Added mutexes to signal_handler and thread
Switched signal_handler to use a stack for the threads so the cancellation is performed in order.
Added cancellation functionality into writer and digitizer_test so that they actually shut themselves down nicely when cancelled.
Added some verbosity so I can see that things are being cancelled.