Fix MRAA non-root access

[bniedermayr]

This MR fixes some issues related the MRAA API access, as non-root user. The following things have been changed/fixed:

1. Add gpio chardev support for MRAA (switch from sysfs to chardev)
2. Add udev rules that adjust permissions and assign required groups to all interfaces
3. The PWM pins require a more special handling and need to be pre-allocated during boot. This avoids timing problems when exporting those pins the first time via MRAA (see issue #29).
4. Create an example (non-root) user that has been assigned to all required groups.
5. Fix MRAA pinmux handling for pin 20