Adding support for 16 channel PWM

Initial commit to add support for the 16 channel PWM expansion module.
It can be called via the ReefAngel.PWM.Set16ChannelValue() method
similar to the ReefAngel.PWM.SetChannelValue() method for the 6 channel
expansion.

To activate it in the code you need to have "ReefAngel.Add16ChPWM"
somewhere in your .ino file even commented out, it will add support.
These expansion channels can also be used as DCPump channels, but the
user should be aware that the 16 channel module only puts out 5V PWM, so
some modifications must be made to control 10V analog pumps like Tunze
Streams.