Changes are:
- added OneWireError exception and used where errors can occur
- renamed read/write functions to use same names as C _onewire funcs
- read_bytes is now read, write_bytes is now write
- add ability to read/write DS18B20 scratch pad
- rename start_measure to convert_temp (since that's what it does)
- rename get_temp to read_temp (consistency with other read names)
- removed test function
