Improve DS18B20 device driver

[mikaelpatel]

1. Allow sensor polling instead of delay time based wait for conversion to complete.
2. Allow relevant commands to be called without MATCH_ROM i.e. in SEARCH_ROM sequence.
3. Add support for parasite powering of devices.
4. Refactor and add sensor collection level; i.e. DS18B20 would handle the sensor collection level and DS18B20::Device would handle the individual level.

DS18B20 sensor(owi);
sensor[0].temperature();

[mikaelpatel]

1 and 2, see commit 0e82888.