docs: function servicesAll (callback) {
      // callback must be a function
      assert(typeof callback === 'function', 'servicesAll callback must be function')
      // call callback for each service
      return Promise.all(_.map(_.keys(getServices()), name => {
          // get service
          var service = getService(name)
          // call callback and return promise
          return callback(service)
      }))
  }