Updated basic TD structure
Updated animation engine.
Updated gitignore : now ignoring .idea/
CompileOption : jdk 1.6 => jdk 1.7
Renamed AnimatedElement to Sprite and GameElement to DrawableElement.
GameEngine : Updated Animation class.
GameEngine : Replaced int by float for positions in Animation and Sprite classes.
TowerDefense : Updated AbstractUnit, Tower and Monster classes.
TowerDefense : Created a TowerFactory to build tower objects.