fix log size problem

Author: shenli.cs

.npmignore

@@ -0,0 +1,2 @@
+example
+qrcode.png

lib/index.js

@@ -105,9 +105,13 @@ var QRCode = React.createClass({
       var image = document.createElement('img');
       image.src = this.props.logo;
       image.onload = function () {
-          var dx = size / 2 - size * 0.1;
           var dwidth = size * 0.2;
-          ctx.drawImage(image, dx, dx, dwidth, dwidth);
+          var dx = (size - dwidth) / 2;
+          var dheight = image.height / image.width * dwidth;
+          var dy = (size - dheight) / 2;
+          image.width = dwidth;
+          image.height = dheight;
+          ctx.drawImage(image, dx, dy, dwidth, dheight);
       }
     }
   },