Adding tests that invalidate the right for en passant.
En Passant now allowed for both white and black. For #3.
Now taking the pawns properly when doing en passant capture.

Weird, heads-up bug: when GameArbiter checks for bugs in checks after the move
is made (for setting flags), a NullPointerException is raised for EnPassant filter.
This shouldn't happen since the call to board.getPiecePositions should have
reflected the move already.