Improved the "!queens" command
Improved the "!queens" command

It is now hardened against DOS by supplying high numbers (greater than 25 for
now). This is to remedy numberten/zhenya_bot#19

Also this introduces pretty printing of the chess board for boards up to 12
rows/columns according to numberten/zhenya_bot#20. For non monospaced fonts this looks a bit ugly though :(

numberten/zhenya_bot