code: function(publicKey, privateKey, callback) {
          this.publicKeyExists(publicKey, function onPubKeyExistsResponse(err, exists) {
              if (err) {
                  return callback(err);
              }
  
              if (exists) {
                  return callback(new Error(
                      'Public key `' + publicKey + '` already exists'
                  ));
              }
  
              this.editPublicKey(publicKey, privateKey, callback);
          }.bind(this));
  
          return this;
      }
docs: Add a public/private key pair
  
  @param {String} publicKey Public key you want to add
  @param {String} privateKey Private key for the public key
  @param {Function} callback
  @return {ImboClient}