Enhancement: Add functions for uci get/get_all/set/commit/...

[Horrendus]

Use cases for that would be to automatically configure e.g. network or wireless settings of an OpenWRT device. Or some users might use UCI to store their own additional system settings.

Maybe due to the big difference in use case for these calls compared to e.g. get_all_connected_devices the "direct" UCI requests should not be accessibly via the main API (to not clutter it too much).