Improved infinite program handling in Maze:
- A check for empty blocks is done before execution.
- Programs with infinite loops now run much faster.
  An example of such a program would be one in which PegMan
  turns right forever.
I also simplified the code to set Maze.stepSpeed.
More changes to speed up infinite programs:
- Make maximum number of steps dependent on level (50 * level) instead
  of always 1000.
- If program times out, skip the normal one second delay before displaying
  feedback.
Consider adding special feedback message for timeout in future.
Merge pull request #25 from espertus/mooc

Improvements related to infinite loops and empty blocks