lib/ds18x20: Added functions isDriverLoaded() and loadDriver().
test/unit/driver-* -> test/unit/ds18x20-* since the added functionality to ds18x20 is only reflecting the drivers functionality transparently.
test/unit/lister-get-test: Updated test implementation.
test/unit/lister-get-test -> test/unit/ds18x20-list-test: Renamed test and changed to call the ds18x20.list() function instead since it's transparently calling lister.get()