Create valueline.js

增加双数值轴曲线，可以绘制轴反转的曲线：
用法如下：
    var line = new Line("chart"),
        tmpXData = [],
        tmpYData = [];
    for (var i = 0; i < 1000; i++){
      tmpXData.push(i);
      tmpYData.push(Math.random());
    }
    var source2 = [{name:"data1",
      visible: true,
      data: tmpXData },
      {"name":"data2",
      data: tmpYData}
    ]
    line.setSource(source2, {line: "line", x: "data1"});
    line.render();