Written base class for a sensor

Added makefile based on scons utility.
Added skeleton of VoltageSensor class

Added pin information to Sensor base class
Implemented test trigger() method

Instantiated class in the main procedures. The trigger method now just prints the row value obtained by
analogRead().
Improved `VoltageSensor::trigger()` method

`VoltageSensor::trigger()` method now saves data and gives as output the
correct voltage/current values computed with the parameter specified in
the constructor. Changed pin paramenters in constructor and attributes (now two).
Added GNU GPL v3 header to files
Fixed bug in conversion from analogRead() output
Merge branch 'master' into voltage
Merge branch 'master' into voltage
Added some comments and documentation to functions
Possible fix for issue #1 (to be tested)

Added delay after each analogRead() call for battery voltage
Resistor values changed and added as constant parameters

Added eventual debug output (if defined DEBUG_0) in
VoltageSensor::trigger()