Merge pull request #91 from BU-CS673/production

Production
Merge pull request #106 from BU-CS673/production

Production
Merge pull request #107 from BU-CS673/backend

Backend
Added Tile class

Tiles must have a Terrain, and can hold one Modifier and one
StationaryElement. Wrote unit tests for all public methods. Added getX()
and getY() overrides to StationaryEntity that kick in if a parent Tile
is set, which use grid location instead of xPos or yPos to determine
world location.
Merge pull request #112 from BU-CS673/Aaron

Added the Tile class, which should make map parsing easier.
Merge pull request #127 from BU-CS673/production

Production
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.
Merge pull request #130 from BU-CS673/Aaron

Aaron
Merge pull request #125 from BU-CS673/backend

Backend -> production