fix - drag durring opponent's turn

[MaligynK]

Sorry for my language.
Bug - When player drag piece durring opponent's turn, piece appears on board after opponent's move.
Decision - It is check position on source dragged.

[MaligynK]

https://github.com/oakmac/chessboardjs/pull/42/files - other decision for this bug

[LeBlanc666]

var board2 = new ChessBoard('board2', {
draggable: true,
dropOffBoard: 'trash',
sparePieces: true
});
$('#startBtn').on('click', board2.start);
$('#clearBtn').on('click', board2.clear);

[user14159]

Thanks andrei-markeev and MaligynK for this fix. Now is there a way to trash the dragged piece if it's been captured by the opponent in the meantime?