for more uniform initialization of Agents.

Improved GUI with commands to list objects and agents.

Fixed GUI speed control (delay slider), narrowed the
range and increased the resolution.

Added three new image files and image credits file.
Better image for dirt

    1.  Unquoted Tk graphics section and created a main window for the GUI.

        Refactored code for graphical user interface, splitting EnvFrame
	into EnvGUI (main Tk window), EnvToolbar, and EnvCanvas.

    2.  Disabled binding of Mouse-1 and Mouse-2, which don't do anything 
	useful.

    3.  Implemented the add-object pop-up menu.  Defined object_classes for
	each kind of environment.

    4.  Added images for ReflexVacuumAgent and Dirt, and a get_image
	method.  get_image caches images in a dictionary to prevent
	them from being garbage-collected and avoid repeated reading
	of the same files.

        Created image files for reflex vacuum agent and dirt.

B.  Changes to XYEnvironment:

    1.  Implemented find_at method.

    2.  Implemented move_to method including bumping.

C.  Other changes -- to support bumping, defined the Obstacle class,
    and made Wall a subclass of Obstacle.  A bump occurs when an agent
    tries to move to a cell occupied by any kind of Obstacle.