Version 0.1.06

Backend changes:
* Added a `vision` column to the `enemy` table.  Defines monster sight radius.  Fixes #53
* Whenever the player moves, all monsters now check to see if the player is visible, and become angry if so.  Fixes #53
* Monster vision is rounded--that is, they cannot see their full vision diagonally, because each tile is ~1.47 distance.
* Modified the check for attack range to allow diagonal attacks.  It rounds the distance rather than flooring.