Add hardware nodes for Beaglebone Black: analogue in; discrete in/out; pulse in/out

A set of 5 hardware I/O nodes for the Beaglebone Black, based on Bonescript.

The BBB should be running with the as-supplied device tree compiled into the kernel. This
code is not currently aware of dynamically loaded device tree overlays, and in particular
the list of pins presented for each node is based on the default. You might be lucky though...

You need to be root to use bonescript, an error message will be logged if you are not.

This code has been tested with the latest node-RED from github as of 2014-02-27T20:00
using bonescript 0.2.4 running under node v0.8.22 on Angström
Linux 3.8.13. The BBB was reflashed with the 2013.09.04 image, updated using opkg upgrade.

Note that you do not need to add bonescript to node-RED's global context (in settings.js) to
use these nodes, but no harm should arise if you do.

Currently there are no checks against assigning the same pin to multiple nodes. Don't do it!