Store/manage all instances of GPIOPin

[EnotionZ]

Since gpio.open acts like a Factory pattern for a discrete number of pins, there's no reason why we shouldn't store the GPIOPin instances internally. The benefit is that gpio.close will get all the benefit of contextually closing from the instance (port.close) like killing the filewatcher or whatever else we may add in the future.

The other benefit is if a user happens to open a port that has already been opened, then we can return the GPIOPin instance immediately without trying to close and reopen.

[rzr]

Maybe we need an other object to relate system configuration.

IMHO, It would make sense to keep gpio as a "minimal" driver to get access to sysfs.