. Adds the files that are used to test the I2C, UART, SysTick, lcd, Stepper libraries
. Minor update to the Makefile to support the test files
. Preliminary code for Queue-based SysTick timer callback managment

. SysTick_Handler does NOT call callback functions directly; rather it pushes the pointers to a queue.

  *** This fixes the limitation of NOT being able to call interrupt based
      functions such as I2C, LCD from within the timer callback function
. SysTick.h now declares the queue as extern

. SysTick.c implements the acutal definition of the queue

. Minor fixes, clean-ups and renaming in SysTick.c and Makefile

. test_SysTick.c now modified to test the new feature