Main modifications:
* All time functions are now in ptime.[hc]
* Added a simple barrier in pbarrier.[hc]
* Added an example to show the use of pbarrier
* Added a test directory
* Added an intermediate function for the thread: now the
  user function has prototype void fun(void).
* Removed the necessity to specify an index for wait_for_period()
  and wait_for_activations()
* Added a pthread_cleanup handler in the function
* Prepared to removing the need to specify the index i in the
  task_create()