Adding ISO-8601 format to JsonStringMessageDecoder

JsonStringMessageDecoder currently passes the value of camus.message.timestamp.format directly into SimpleDateFormat for date parsing. This change adds a special value for camus.message.timestamp.format of "ISO-8601" that will instead use joda-time's DateTime constructor to parse ISO-8601 dates. This is convenient when you know your date format conforms to ISO-8601, but has different levels of precision (some timestamps have millis, some only seconds, etc).