code: function(evname, callback, context) {
      var self = this, args = arguments;
      this.on(evname, function() {
        callback.apply(context, arguments);
        self.off.apply(self, args);
      });
      return this;
    }
docs: Bind event listener and only fire once
  
  @param {string} name of event
  @param {function} callback if event is triggered
  @param {object} context in which callback is called
  @return {this}