Added Tile enum

including FLOOR, WALL, BOUNDS(null pattern), STAIRS_UP, STAIRS_DOWN, 
and UNKNOWN

plus isDiggable() and isGround()
added World class

World is composed of a 2 dimensional array of tiles
added WorldBuilder class

populates a 2 dimensional array of tiles by randomly filling with FLOOR
and WALL and then smoothing so that tiles are more likely to match their
neighbours
updated PlayScreen to call WorldBuilder and show World

added a constructor, createWorld method, and displayTiles method which
calculates which world tiles to draw on terminal screen
Merge branch 'master' of https://github.com/gdbing/veins-rl/