Concept: mt scheduler with events

This is just a proof of concept. Thus I placed it in /examples/multi-threading/mt-scheduler

Compiles without warnings and the examples work on native linux

- Non preemtive mt-scheduler (context switch during irq not allowed for this implementation)
- contiki process -> event -> smt_thread works
- smt_thread -> event -> smt_thread works
- smt_thread -> event -> contiki process work 
- pausing smt_thread works (sleep ... wakeup via ctimer + event)
- minor changes in mt.h, mt.c (mt_current(), mt_thread -> list node)
- Additional MEMB for extended event data structure required (OUCH!)
- Increased stack size for native linux mt ... no tests with MSP430 so far
- mt_thread has to exit on its own! (no kill allowed ... see smt_sleep autovars ... maybe memb_alloc instead?)

To be done:
- PROCESS_BROADCAST handling (sends to all processes, also mt_scheduler_process -> problem!)
- PROCESS_EVENT_EXIT handling (sends to all processes, also mt_scheduler_process -> ignore event?)