Rewrite seven seg lib into C
Rewrite the C++ SPI lib from Arduino into C
Add arduino C core files from Arduino project
Correct display_7_seg to use C lib and add a Makefile
Merge branch 'dev_seven_seg_c' into dev_sched
Move SevenSeg tab to c file instead of header
Re-inline some functions in SPI
Add priority handling in task.

The most priority task will be included before the others in the task list.
If two tasks have same priority, the first one added will be more priority than
the following.
Update test_sched project to use with new Sched
Add pin_arduino.h file from Arduino core