SchedulerARMAVR

Proposal for a new version of the Scheduler.h, now including asm code
for AVR and 3 macro to facilitate atomic and concurent situation
handling
Revert "SchedulerARMAVR"

This reverts commit 3087d469660ac26b1bf241c04522fb62fc97feb3.
SchedulerARMAVR

Proposal for including a new version of the Scheduler.h in the Arduino
libraries, including asm code for AVR task switch. resulting file
compatible with both ARM and AVR.
Merge pull request #1212 from fabriceo/scheduler

Scheduler ported to AVR