forcing path to be None so as to show this bug.

HI; this is a bug report, not really a pull request.

I noticed that if the path is not found, the _drawX function throws an error.   It says 'PixelAccess' object is not iterable.  I don't know of a trivial fix, but I thought I'd ask you first, just in case you do.

Cheers
  Rob


```
thunderrabbit$ ./solve_maze.sh mazes/mazefeb2014.png 
INFO: Loaded image 'mazes/mazefeb2014.png' ((2480, 3508) pixels).
INFO: Solving...
ERROR: No path found.
Traceback (most recent call last):
  File "solve.py", line 169, in <module>
    solver.solve()
  File "solve.py", line 85, in solve
    self._drawX(self.pixels, self.START)
  File "solve.py", line 100, in _drawX
    x,y = pos
TypeError: 'PixelAccess' object is not iterable
```