gpio-admin support

[Hypnopompia]

I'd love to see this module support gpio-admin for exporting and unexporting pins instead of needing to be root.

An example of another node gpio module that does this: https://github.com/rakeshpai/pi-gpio (however that module doesn't seem to support interrupts like this one does)

[fivdi]

There is a section in the read-me describing an alternative technique for handling superuser issues. Briefly, you can export and unexport pins as the superuser and run your application as a non-superuser.

That being said, if onoff played well with gpio-admin it could take advantage of gpio-admins ability to configure pull-up and pull-down resistors for GPIO pins.

[fivdi]

As of commit c3778cf onoff plays well with gpio-admin on the Pi. onoff doesn't use gpio-admin internally as gpio-admin isn't available for the BeagleBone. It is however now possible to use gpio-admin for export/unexport and pull-up/pull-down resistor configuration. Please see the readme for more details.