restore support for DS1820

- the DS1820 (circa 1998) is still in use in some temperature
  networks, and can be identified by the double height PR35 package,
  and a scratchpad COUNT_PER_C register value that changes during
  operation, but not by the family code in the device address,

- fixes a regression introduced in e1cdffb, resulting in sawtooth
  temperature graphs for a linear temperature change, caused by using
  a fixed COUNT_PER_C register value for DS18S20 and therefore DS1820,

- tested with both DS18S20 and DS1820 sensors.
increase resolution for DS1820

The DS1820 can achieve a higher resolution than 12-bits, because of
the much larger values seen in the COUNT_PER_C register.

The previous change from floating point to fixed point 12-bits lost
useful data from the DS1820.

Change the fixed point scaling factor of the raw temperature returned
by the calculateTemperature method.

Change the scaling factor of the getTemp method.  Will break programs
that have relied upon the previous scaling factor.
Merge pull request #18 from quozl/master

restore support for DS1820. Many thanks to Quozi