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 #128 from BU-CS673/backend

Backend