Fixed a bug with calculating safe positions

The bug was noted in numberten/zhenya_bot#20. Reason was using a wrong reference
as starting point for further checking for safe positions. Sadly this fix
introduces a massive runtime penalty. I am not sure about the reason for that.
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 :(
Merge pull request #21 from keneanung/Queens

Adapted the queens module to fix open issues. Closes #20 and #19.