Save and load

Implemented save and load for a single player in the game. Can only save
to one profile at the moment.
Merge branch 'save-load'

Conflicts:
	src/pvz/GUI/GameController.java
	src/pvz/level/GameModel.java
	src/pvz/level/Player.java
	src/pvz/unit/OffensivePlant.java
	src/pvz/unit/Zombie.java
	src/test/GameModelTest.java
	src/test/PlayerTest.java
Merge branch 'master' of github.com:tmzhuang/pvz-reloaded