Adds documentation to the RedBot Arduino library.

Only documentation to the classes public members and methods was
added as the implementation files are sufficiently documented for
people who really want to dig.

Signed-off-by: ZachWick <zach@zachwick.xxx>
Constrain PWM duty cycles to [0-255]

Signed-off-by: ZachWick <zach@zachwick.xxx>
Update documentation.

Signed-off-by: ZachWick <zach@zachwick.xxx>