on (event, method, context) {
    // TODO: we could add options like 'once'
    _on.call(this, event, method, context)
  }
Subscribe a listener to an event.

    Optionally, a `priority` can be provided to control the order
    of all bindings. The default priority is 0. All listeners with the
    same priority remain in order of registration.
    A lower priority will make the listener be called later, a higher
    priority earlier.

    @param {String} event
    @param {Function} method
    @param {Object} context