added some documentation to README

* documentation about native networking
* section about examples
* markdown formatting
removed unused (and ambiguous) definitions of active_thread and sched_threads in kernel_init.c
moved prototype for sched_register_cb() from C file to header
added documentation for sched.h
re-added the scheduler callback (the actual call was missing)
change python shebang to python2 -> python is mostly python3
fix priority comparison in sched_switch
fix minor typo in status output
minor changes according to 7a2cc4ae096a90308c026dd857e19b4489b162cf
clean up of ifdefs in transceiver module
fix for #117 and position of preprocessor directives
some debug enhancements
removing residual breaks in switch-case statements due to #ifdefs
added documentation for sched.h
Improving sched.h documentation
fix printfs in hwtimer_cpu.c
removed duplicated paragraph
changed default stacksize

* the default stacksize no longer set for a thread using printf
* the stacksize for the main thread therefore adds the necessary space
make transceiver make use of the default stacksize
Implement simple echo request/reply