The main routine now uses the two detectors and the RPi camera to
detect and photograph birds.  The editable parameters have been exported
to parameters.py.
Corrected a bug in passive_IR.py (pin setup at incorrect time).  Added
some documentation to usonic.py.
Minor changes:
 * Updated README.md
 * Added more parameters to parameters.py, organized them better
 * Made main.py an executable script
Renamed "io" to GPIO in passive_IR.py to be consistent with usage in
usonic.py.
Refactored usonic.py to contain only one definition, that of
the Ranger class.  Ranger operating parameters are now passed to the
class constructor.