code: function(eventName, callback) {
        if (!callbacks[eventName]) {
          callbacks[eventName] = [];
        }
        callbacks[eventName].push(callback);
      }
docs: @method on
  @memberof salep
  
  @desc
  This function enables adding callbacks to events. For one specific
  event there may be many callbacks.
  
  @param {String}    eventName Event name to add callback
  @param {Function}  callback  Callback
  
  @example
  salep.on('fail', function(testCase) {
  console.log(testCase.name + ' has failed!');
  });