code: function(options) {
          var params = [],
              opts = options || {},
              transform = 'sharpen';
  
          if (opts.preset) {
              params.push('preset=' + opts.preset);
          }
  
          if (typeof opts.radius !== 'undefined') {
              params.push('radius=' + opts.radius);
          }
  
          if (typeof opts.sigma !== 'undefined') {
              params.push('sigma=' + opts.sigma);
          }
  
          if (typeof opts.gain !== 'undefined') {
              params.push('gain=' + opts.gain);
          }
  
          if (typeof opts.threshold !== 'undefined') {
              params.push('threshold=' + opts.threshold);
          }
  
          if (params.length) {
              transform += ':' + params.join(',');
          }
  
          return this.append(transform);
      }
docs: Sharpen the image
  
  @param  {Object} [options]
  @param  {String} [options.preset]         Name of a defined preset to use
  @param  {Number} [options.radius=2]       Radius of the Gaussian operator in pixels
  @param  {Number} [options.sigma=1]        Standard deviation of the Gaussian, in pixels
  @param  {Number} [options.gain=1]         Percentage of difference between original and blurred image
  that is added back into the original
  @param  {Number} [options.threshold=0.05] Threshold in pixels needed to apply the difference gain
  @return {Imbo.ImageUrl}