Added #threaten? to knight.

Corrected #eql? on value objects.
Changed board to use [rows,cols] instead of [width,height].

Reduces ambiguity (I keep getting it flipped around in my head).
Besides, this change also brings it into line with standard
matrix notation.  (Yay, linear algebra.)
Created class to generate legal pairings of knights.
Refined pairings quite a bit.
Created hackneyed commandline executable.

And NI!