Corrected nfc_device_lookup() - fixes #6

Now the vote machine requires PyUSB 1.x, which much be installed using
pip instead of installing the python-usb Ubuntu package.
Added the PyUSB version note in the README
Changed LED configuration in vote.ini

The LED configuration key is now the index of the associated value in
the vote values list, instead of the value itself.
This avoids invalid configuration keys.

E.g.: value=-1,1 => LED keys will be 0 and 1 instead of -1 and 1