Construct a grant manager.

@param {Config} config Config object.

@constructor
function GrantManager (config) {
  this.realmUrl = config.realmUrl;
  this.clientId = config.clientId;
  this.secret = config.secret;
  this.publicKey = config.publicKey;
  this.public = config.public;
  this.bearerOnly = config.bearerOnly;
  this.notBefore = 0;
  this.rotation = new Rotation(config);
}