Only allow deploying in one of your towns.

Future refactoring and improvements::

* combine OgreTown and OgreUnit into OgreObject, then subclass
* move tiles to be centered?
* actually name units/heros
* change unit color so more opaque = more alive
* improve battle
  ** use diff hero types
  ** rock paper scissors kind of attack, also diff color or shape
     for the diff types?
  ** so wiz > fighter, amazon > wiz, fight > amazon, hawk general
  ** or maybe amazon is generalist, whatever