New input process: signed to unsigned

Unsigns a value received as signed. This allows for the recombination
of longs transmitted as signed ints.

This commit creates arg type None, for input processes that don't need
any argument, like this one, or allow positive / negative.

I think this addresses this issue: http://openenergymonitor.org/emon/node/1964

Please review, at least the change in input_model.php. It seems to work here, but perhaps could it be simplified.