code: function(options) {
          options = options || {};
  
          if (!options.width || !options.height) {
              throw new Error('width and height must be specified');
          }
  
          var params = [
              'width=' + toInt(options.width),
              'height=' + toInt(options.height)
          ];
  
          if (options.mode) {
              params.push('mode=' + options.mode);
          }
  
          if (options.x) {
              params.push('x=' + toInt(options.x));
          }
  
          if (options.y) {
              params.push('y=' + toInt(options.y));
          }
  
          if (options.bg) {
              params.push('bg=' + options.bg.replace(/^#/, ''));
          }
  
          return this.append('canvas:' + params.join(','));
      }
docs: Puts the image inside a canvas
  
  @param  {Object} options
  @param  {Number} options.width  Width of the canvas
  @param  {Number} options.height Height of the canvas
  @param  {String} [options.mode] Placement mode: "free", "center", "center-x" or "center-y"
  @param  {Number} [options.x]    X coordinate of the placement of the upper left corner of the existing image
  @param  {Number} [options.y]    Y coordinate of the placement of the upper left corner of the existing image
  @param  {String} [options.bg]   Background color of the canvas, in hex-format
  @return {Imbo.ImageUrl}