Scheduler ported to AVR

Hello
I ve pushed SchedulerARMAVR in the libraries folder of the scheduler branch.
please review if appropriate and relevant.

this library is 100% compatible with original Scheduler.h and just add asm code for AVR platform, to provide the taxt context switching also for AVR.

also included 3 macros for dealing with atomic and concurent situation
yieldPROTECT() yieldUNPROTECT()
yieldATOMIC

remark : thats my first contribution on github, hope I did it the right way :)
cheers