Add Knight capture test.

This adds a needed test of my capture and canCapture methods, which were
defined in the abstract ChessPiece class. Knights capture the same way
they move, and hop over other pieces, so this should work fine with the
default functions given in ChessPiece.
Add file & rank Pawn constructor.
Replace attribute access with method calls in Pawn.

I was directly accessing the row and column instead of using the
accessors I made.