Fixed Tile, added Entities, removed Modifier

Added:

* Swamp
* Water
* DeepWater
* Rubble
* Crater
* Wall
* Mine
* Pillbox
* Base
* Bullet
* Man

* Added documentation to the Tile class.

* Created basic unit tests for constructing these concrete entities,
which should provide a foundation for future tests.

* Removed the Modifier class and converted Road to a Terrain.