PWM default state change and a bugfix

1) Changed default PWM channel state after initialization to be low (0%
duty cycle)
2) Fixed a bug which caused random channel state after PWM
deinitialization
Added restart function

Added SFP function "restart" which restarts the device. After calling
this command, the user should close COM port in less than one second
(and not open it during that period), otherwise the operating system
might not release COM port resource properly and a physical USB
reconnect will be required.