ImageDimensions constructor.
@param {String}    assetPath         the file path from sass
@param {Map}       registeredAssets  registered assets from sass
@param {Function}  callback          callback function to deliver results
function ImageDimensions (assetPath, registeredAssets, callback) {
    sassUtils.assertType(assetPath, "string");
    sassUtils.assertType(registeredAssets, "map");
    var self = this;
    var getPath = self.checkImagePath(assetPath.getValue(), registeredAssets);
    getPath.then(function (success) {
      var imageDimensions = self.getDimensions(success);
      imageDimensions.then(function (dimensions) {
        callback(null, dimensions);
      }, function (err) {
        callback(err, null);
      });
    }, function (err) {
      callback(err, null);
    });
  }