Added support for an Origin header in STUN messages.

For WebRTC there are instances where it may be desirable to provide
information to the STUN/TURN server about the website that initiated
a peer connection. This modification allows an origin string to be
included in the MediaConstraints object provided by the browser, which
is then passed as a STUN header in communications with the server.
A separate change will be submitted to the Chromium project that
uses and is dependent on this change, implementing IETF draft
http://tools.ietf.org/html/draft-johnston-tram-stun-origin-02

Originally a patch from skobalt@gmail.com.

(https://webrtc-codereview.appspot.com/12839005/edit)

R=juberti@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/41409004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@8035 4adac7df-926f-26a2-2b94-8c16560cd09d

AUTHORS

@@ -9,6 +9,7 @@ Christophe Dumez <ch.dumez@samsung.com>
 Colin Plumb
 Eric Rescorla, RTFM Inc.
 Giji Gangadharan <giji.g@samsung.com>
+Graham Yoakum <gyoakum@skobalt.com>
 James H. Brown <jbrown@burgoyne.com>
 Jie Mao <maojie0924@gmail.com>
 Luke Weber
@@ -21,6 +22,7 @@ Rafael Lopez Diez <rafalopezdiez@gmail.com>
 Ralph Giles <giles@ghostscript.com>
 Robert Nagy
 Ron Rivest
+Ryan Yoakum <ryoakum@skobalt.com>
 Sarah Thompson <sarah@telergy.com>
 Saul Kravitz <Saul.Kravitz@celera.com>
 Silviu Caragea <silviu.cpp@gmail.com>