mainly documentation and removed some warnings
- schedule timer now really ticks with millisecond precision
- introduced scheduling with simple round robin
- fix for gptimer_get( ... ) bernhard pointed out in revision:
https://github.com/bernhardbreuss/OS/commit/9e91953ca6b78200c0ddf2251b193342e2b3d43d
- irq generic module offers possibility to register handlers for all interrupt lines of MPU Subsystem
program_manager module uses this interface to register its schedule interrupt handler for the schedule timer

- open TODO's:
-> ask the manager in "src\kernel\ipc\ipc.h"
-> _process_manager_irq_schedule_handler(void), find a better place for the code mentioned by the TODO inside the function