Bugfix: Using FileGpioConnectionDriver as non-root user

Hi,

I am currently working on a private project using the FileGpioConnectionDriver. When starting the c# program as non-root user I always get a UnauthorizedAccessException - even though the user is a group member of *gpio*. It seems that the system needs some time in order to set the appropriate file permissions to the new created gpio?/direction file.

```bash
#!/bin/bash
echo "17" > /sys/class/gpio/unexport
echo "17" > /sys/class/gpio/export
#sleep 1 # uncomment this line and it will work
echo "out" > /sys/class/gpio/gpio17/direction
echo "1" > /sys/class/gpio/gpio17/value
```
When running this as test.sh shell script as non-root user I'll get permission errors.
```bash
./test.sh: line 4: /sys/class/gpio/gpio17/direction: Permission denied
./test.sh: line 5: /sys/class/gpio/gpio17/value: Permission denied
```

Note: I am currently using Raspbian.

Same problem reported here: https://github.com/fivdi/onoff/issues/20

Greetings,
Daniel