Fix timezone detection of parse-iso8601-time-string

Fix timezone detection of parse-iso8601-time-string

* parse-time.el (parse-iso8601-time-string): Fix timezone
parsing.  Add a doc string.  (Bug#25086)
* editfns.c (Fdecode-time): Doc fix.
* emacs-mime.texi (time-date): Add an example for
parse-iso8601-time-string.
* parse-time-tests.el (parse-time-tests): Add tests for
parse-iso8601-time-string.