Added usonic.py: its one function returns estimated distance in cm.
Added averaging function to usonic.py
Made the settling time a named parameter.
Added class Ranger: detects bird presence using ultrasonics.
Added an object-based interface to the PIR sensor.
Added comments.
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.