code: function(claims, secret, alg) {
                  claims = claims || {};
                  claims.exp = claims.exp || jwt._generateExp();
  
                  if (!claims.iss) {
                      throw new Error('jwt:undefined:iss');
                  }
                  if (!claims.aud) {
                      throw new Error('jwt:undefined:aud');
                  }
  
                  return jwt._generate(claims, secret, alg);
              }
docs: JWT-HmacSHA256 generator
  http://self-issued.info/docs/draft-ietf-oauth-json-web-token.html
  @param  {Object}                                 claims Specific claims to include in the JWT (iss, aud, exp, scope, ...)
  @param  {String} secret                          String with the client assigned secret
  @param  {Object} [alg='corbel.jwt.ALGORITHM']    Object with the algorithm type
  @return {String} jwt                             JWT string