implemented is_solved for basic board
add remove_board functions; Board_factory keeps track of the next board id instead of Model
changed some stuff
switch the meaning of pos/neg values in relation to sliding a column up or down, pos values now mean up and vice versa for neg
update makefile, use quiet compile messages
add solve_board command; want to add undo command, but need slight restructuring of Board to make it work
add perf.data to gitignore