Support 64 bit trace ID

Documentation: http://zipkin.io/pages/instrumenting.html
Ticket: #11

Author: sokac

src/functions.php

@@ -35,7 +35,8 @@ function is_zipkin_timestamp($timestamp)
      * @return bool
      */
     function is_zipkin_trace_identifier($identifier) {
-        return ctype_xdigit((string) $identifier) && strlen((string) $identifier) === 32;
+        return ctype_xdigit((string) $identifier) &&
+            (strlen((string) $identifier) === 16 || strlen((string) $identifier) === 32);
     }
 }