added .data_by_index(const int) to the Scanner

This gives access to individual data elements so I don't have
to get a whole JSON string to see a particular value.
Major revision - Scanner is now subclass of Sensor

Added a superclass called sensor in the utilities folder.  All my
sensors (i.e. IR scanner, bump detection, etc) to measure objects
will inherit from Sensor.h.  Because Sensor.h is included in the
library with Scanner, it must be modified in all the other sensors
as well.  I did it this way so that the Scanner class can be
cloned with all necessary components from Github.
Merge branch 'master' of https://github.com/zacsketches/Scanner