created placePawn() for some edge cases

when switching pawns with card 11 or Sorry!, we were not checking if
pawns landed on a slide. When starting pawns, we were not checking if
there was an enemy pawn right outside the start space that needed
bumping. I created a function in GameBoard to place a pawn that checks
for cases like these. It is used by movePiece() so any other edge cases
that I didn't think of should be taken care of.