clean up and optimization of ws2812 support

delete req=55 and implemented the functionality with parametrization of
the data payload.
ws2812 support: added flush function

Added ws2812_flush function to write the internal buffer to the led
string without having to pass another rgb value. This simplifies the
programmer interface.
ws2812 support: added two examples

fade_ws2812: Adapted from Softpwm fade
rgb_cycle_w2812: cycles an led string using the internal buffer.