Obstacle: Breakable 'walls'

[AJFaraday]

From issue #2

Now there are some obstacles that are completely impassable 'rocks', use the same pattern to create some walls.

• Walls should have a health property. Which works the same way as cities, but with no owner.
• Unlike cities, they do no damage in return.
• There should be a config for the initial health of walls.
• Until their health reaches zero, walls are impassable.
• When health reaches zero, or less, they cease to be walls, and are once more passable.

[AJFaraday]

Changes in pull request #13. I have, rather frighteningly, asked Twitter for some reviewing.