implement direct pin for beaglebone

I noticed I couldn't use NewDirectPin with a beaglebone due to not implementing the full interface.

I went ahead and took a first crack at getting it working. This should also make button and makey_button work.

Replaces https://github.com/hybridgroup/gobot-beaglebone/pull/2
My bad! That's what copy/paste gets me. Should be all good now.