Adding Pieces + Moves - Pawn
Adds pawn moves, color state, has_moved state
Adds Board Class
Fixes piece movement bugs
Determines checks and raises move errors
Adds #dup and #move_into_check?
Adds #move! and #move raises CheckError
Adds #filtered_moves and #checkmate?
Adds Game class, adds #place_pieces
Adds Board::render and Board::render_char