Consider using /dev/gpiomem for rootless use

[techniq]

Might be worthwhile to use /dev/gpiomem (and fallback if not available) which would allow rootless use of the library. See https://github.com/n8henrie/node-rcswitch-gpiomem for another library that does this, and http://wiringpi.com/wiringpi-update-to-2-29/.

[FlorianWendelborn]

This is one of the gpis-related things preventing me from using pm2 with a project that uses this. Would be awesome to see it fixed.