Opto Endstop v3.0

This is almost identical to the Opto Endstop v2.0 board, except that we switched connectors to a standard Molex header typically used for CDROM audio signals. These cables are cheaper, and the headers have a smaller footprint than the RJ45 jacks. They are also less likely to cause confusion, and a whole bunch of other nice things.

This board will be released in tandem with the Stepper Motor Driver v3.0 board that has matching connectors.