CAPWAP: Add some new Message Element Type (RFC5415)

* AC Timestamp (6)
* Add Station (6)
* CAPWAP Local IPv4 Address (30)
* CAPWAP Local IPv6 Address (50)
* CAPWAP Transport Protocol (51)
* MTU Discovery Padding (52)
* ECN Support (53)

Change-Id: Ib8b2768d5df4b0e1b5b8b5bd28ae66e825020205
Reviewed-on: https://code.wireshark.org/review/3986
Reviewed-by: Michael Mann <mmann78@netscape.net>
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Anders Broman <a.broman58@gmail.com>