14 25 sysfs permissions

[aSmig]

Fixes #14 and #25 by adding file modes which better match sysfs file permissions

Fixes subsequent permission error when export is successful but sysfs tree buildout is not complete before attempting to use the new value FH. Slower systems may need a longer delay. Looping with os.path.exists() would likely be a better fix.

[Gadgetoid]

Okay I have added the filemode patches to #31 and https://github.com/vitiral/gpio/tree/python-2.7

In lieu of sleep or waiting on a specific gid, I think we should - as mentioned above - just retry with a delay, and fail after N retries. I think that's likely to work reasonably in all cases and avoid edge cases and regressions.