Wait for sysfs node to ready before access (API BREAK) #16
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With current versions of sysfs-gpio on Raspberry Pi, the first run of the application would fail (Permission Denied on SYSFS_GPIO_VALUE_PATH); but the second time (or if the pins were already exported); the library works fine.
As it turns out, on Raspberry Pi (Raspbian Jessie); it takes some time for the sysfs nodes under the GPIO pin to have the correct permissions assigned if you're not running your application as root.
Doing a time.sleep(0.5) works, but is dirty. This branch attempts to do it the "Right Way"; BUT that means Controller.alloc_pin() returns a deferred... which breaks existing code.
On the other hand, the raspberry pi may be the most commonly used / available python+GPIO hardware target so having the library work there is good.