Improve demo page look and feel

This commit styles the demo page so that it looks a little better.

I've also removed the MIT.LICENSE from the test directory.
Add grid instance variable to the grid artist

This is purely a refactor commit, the behavior remains unchanged.

This commit changes the way the Grid2DArtist works. It used to accept a
grid each time the draw() function was called. Now, the grid is
specified on initialization and the draw() command just uses the
instance variable.

This change is made because we need to add click-handling to the artist,
and for that we need to have a reference to the current grid.
Move grid artist cell height and width to instance variables

This commit moves the calculation of the cell height and width as
instance variables. This is now possible because the grid is specified
to the constructor.
Add missing row to demo grid

The demo grid was meant to be 10x10 but was actually 10x9. This commit
adds the missing row (to both the demo and README).
Add interactive grid

This commit adds the ability to add and remove walls by clicking on
cells in the grid. This dynamically updates the path finding and
redraws.

To make this happen I've added the ability for the artist to handle
click events.

Via a callback, the demo.js can now redraw and find a new path each time
a cell is clicked.