PWM Support

-> AutoDetection for Rapberry B+
-> Adding PWM support to Board
-> New PwmPin

Code is tested on Raspberry, but no unittest yet.

Some API design questions became evident.
-> How is the interface of a PwmPin? (duty_cycle, frequency, when to start/stop, etc...)
-> Should _set_analog_mode and _set_pwm_mode exist?
-> Should _set_mode take care of both?