Previously, the return value when a tag was gone was the negative ID.
However, this isn't the most effective to process later as a negative
number uses two's complement, changing all lower bits in addition
to the sign bit.
Change this to just set the sign bit instead. This still produces
a negative number, which is what we want to check to konw if the tag
is new or went away, but maintains the lower bits so the ID can be
retrieved with a simple bit-wise and.
Add a new class to handle the RFID module and associated data
in order to track active tags.
Update main with RFID initialization and a minimal usage to
check it is working as expected.
pinMode most of what could be useful in the main and put modules
LOW to minimize power draw before the regulator is ON.
Add the skeleton for the setup, main loop and battery level checking.
