Added Traderable interface to Transactions.
Players can now trade with Traders encountered in space.
Merge branch 'master' into sam/dev.

Conflicts:
	src/main/java/edu/gatech/gem5/game/controllers/DisplayUniverseController.java
	src/main/java/edu/gatech/gem5/game/controllers/EncounterController.java
last commit before pull from master.
Merge branch 'master' of https://github.com/gem-5/last-adventures into sam/dev
Traders now spawn with inventory goods that they can trade to the player.
Fixed UI bug, where trading with Trader was labelled as Planet.