Revised the MVC framework by introducing a Controller superclass for all controllers.
Merge branch 'master' of https://github.com/gem-5/last-adventures into creston/dev

Conflicts:
	src/main/java/edu/gatech/gem5/game/Planet.java
	src/main/java/edu/gatech/gem5/game/controllers/DisplayUniverseController.java
	src/main/java/edu/gatech/gem5/game/controllers/MarketController.java
Added back button.
Improved the code at @MarketController.java
Implemented sell, not validate sell
merge fix