Adding support of parasite powering of 1-wire devices; DS18B20.

1. Update of the OWI driver to support parasite powering; leaving pin in
output mode to driver the device during operations that require powering.
2. New power parameter to OWI::write() to leave pin on.
3. New method power_off() to turn pin off.
4. Update of DS18B20 to allow parasite mode. Implemented the last
function code, READ_POWER_SUPPLY. Will check if the device is in parasite
mode.
5. Update of the functions that require power on; convert_request,
copy_scratchpad.
6. New method power_off() for DS18B20.