(Timeout): New exception.

(handler): New signal handler.
(TestCase): New class.
(TestCase.setUp): New setup function.  Reset the state of pcl_expect,
	and set up a SIGALRM timer.
(TestCase.tearDown): Cancel the SIGALRM timer.
(TestCase.progress): Re-arm the SIGALRM timer.
(TestCase.assertTimeDiff): New function.