Add Timers to dropoff and package objects

Created a timer class.  Added timers to the dropoff and package objects.
For now they are all just 10 second timers, but in the future it would
be cool to make them depend on how far away the courier is.

I've noticed that a packages and dropoffs are essentially the same
thing.  I should probably merge them into a single class called
Location/Address.
Fix bug in Grid.snapToGrid()

Fixed a bug in snapToGrid().  We should use the position relative to the
grid's position when computing distances to the nearest grid lines.
Fix bug in Package.js and Dropoff.js

Fixed bug in Package and Dropoff.  When snapping the position to the
grid, we need to pass the object's position relative to the grid, and
when setting the position, we need to change it back to an absolute
position.
Change the gridSpacing to 80px

Changed the gridSpacing to 80px to make more room for the art.  Change
the level margin to 20px as well.

Changed the canvas size to allow an whole number of grid blocks in the
canvas.

Also set courier initial position using snapToGrid.
Import AnimatedImage and RegularImage

Import AnimatedImage and RegularImage classes from juego.js.
Add images and animations files

Add images.js and animations.js files which will contain maps called
IMAGES and ANIMS of images and animations.  The only image currently
created is IMAGES.background.

Also add necessary files to courier.html, and add a (rather ugly)
streets background image.
Draw background in main loop

Add a drawBackground function and call it in the main update loop to
draw the background.