Work in progress on issue #18. Implemented the "visible players". Need to be tester now.
Added some debugging tools (str and repr on the players) + added two tests on the visible_players attribute that seem to work. Related to issue #18. some other tests do not currently pass, though, need investigation.
WIP in getting the players back to harming each other. investigation in progress.
Updated the tests and the constants. All GameEngine tests are now passing.
Fix missing field `type` in game state frame.
Merge branch 'gameengine' into server

Conflicts:
	server/game_engine.py
Small fix, related to issue #18. Was not respecting the right format for the visible_players attribute if we want to throw it without changes inside the get_state()
Merge branch 'gameengine' into server