backgroundPositionY 兼容firefox方法

在firefox下jquery的css方法不支持backgroundPositionY属性， 
我们可以通过如下方法解决。 

在使用backgroundPositionY之前调用

https://raw.github.com/brandonaaron/jquery-cssHooks/master/bgpos.js 

github项目来自：

https://github.com/brandonaaron/jquery-cssHooks/blob/master/bgpos.js 

https://github.com/louisremi/jquery.backgroundXY.js/blob/master/jquery.backgroundXY.js 

(function($) {
    // backgroundPosition[X,Y] get hooks
    var $div = $('<div style="background-position: 3px 5px">');
    $.support.backgroundPosition   = $div.css('backgroundPosition')  === "3px 5px" ? true : false;
    $.support.backgroundPositionXY = $div.css('backgroundPositionX') === "3px" ? true : false;
    $div = null;

    var xy = ["X","Y"];

    // helper function to parse out the X and Y values from backgroundPosition
    function parseBgPos(bgPos) {
        var parts  = bgPos.split(/s/),
            values = {
                "X": parts[0],
                "Y": parts[1]
            };
        return values;
    }

    if (!$.support.backgroundPosition && $.support.backgroundPositionXY) {
        $.cssHooks.backgroundPosition = {
            get: function( elem, computed, extra ) {
                return $.map(xy, function( l, i ) {
                    return $.css(elem, "backgroundPosition" + l);
                }).join(" ");
            },
            set: function( elem, value ) {
                $.each(xy, function( i, l ) {
                    var values = parseBgPos(value);
                    elem.style[ "backgroundPosition" + l ] = values[ l ];
                });
            }
        };
    }

    if ($.support.backgroundPosition && !$.support.backgroundPositionXY) {
        $.each(xy, function( i, l ) {
            $.cssHooks[ "backgroundPosition" + l ] = {
                get: function( elem, computed, extra ) {
                    var values = parseBgPos( $.css(elem, "backgroundPosition") );
                    return values[ l ];
                },
                set: function( elem, value ) {
                    var values = parseBgPos( $.css(elem, "backgroundPosition") ),
                        isX = l === "X";
                    elem.style.backgroundPosition = (isX ? value : values[ "X" ]) + " " + 
                                                    (isX ? values[ "Y" ] : value);
                }
            };
            $.fx.step[ "backgroundPosition" + l ] = function( fx ) {
                $.cssHooks[ "backgroundPosition" + l ].set( fx.elem, fx.now + fx.unit );
            };
        });
    }
})(jQuery);