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.