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
This examples fails to compile
```
./beaglebone_direct_pin.go:5: imported and not used: "github.com/hybridgroup/gobot/platforms/beaglebone"
./beaglebone_direct_pin.go:13: undefined: beagleboneAdaptor
./beaglebone_direct_pin.go:14: undefined: beagleboneAdaptor
./beaglebone_direct_pin.go:27: undefined: beagleboneAdaptor
```

you're missing the `BeagleboneAdaptor` instantiation