Pullup on port B disabled
Adjusted startup and cylon sequences

Startup now ramps-up, making it more interesting and quicker.

The cylon sequence is a more robust, though it is more like the knight
rider leds with the leading LED being the brightest. The cylon scanner
has a symmetric pattern with the middle LED being the brightest.
Added macros for atomic operations

ATOMIC_BEGIN and ATOMIC_END can be used to wrap bodies of code that
should not be interrupted. ATOMIC_BEGIN simply disables interrupts
globally, while ATOMIC_END re-enables them and then checks the hardware
interrupt for the accelerometer. If the flag is set, it will call
check_tilt to read the tilt register.