js: extend information on decoding in decrypt

• src/Connection.js: resulting data, if not pure ascii, is base64 encoded in the result message. A further decoding attempt into javascript 'string' will be attempted by default, unless specified at the decrypt() method. The return value 'format' now shows which of the possibilities has been applied. The old boolean 'base64' now turns into format:'base64' if the returned payload is a base64 string after decryption.