Added licensing notice in yd/NOTE.txt file.
Updated bindings, WASD keys now send impulse commands for movement, Q and E
keys send impulse commands for turning.  Also adjusted what digits send.
Game : began work on new movement and turning logic (via 'impulse' commands).
This commit sets up some stub code in client.qc to handle the impulse cmds.
Data : adjusted 'ceiling01.obj' piece to be 0.5 tile units (128 game units)
off the ground -- i.e. making it relative to the ground.
Game : added tile utilities Tile_MiddleCoord() and Tile_FromCoord(), and
reposition player start entities to be at middle of the tile.
Game : define DIR_NORTH,SOUTH,EAST,WEST constants, added Tile_Neighbor() utility.