Advertises dtls/sctp support in capabilities.

libs/strophe/strophe.jingle.js

             this.connection.disco.addFeature('urn:xmpp:jingle:1');
             this.connection.disco.addFeature('urn:xmpp:jingle:apps:rtp:1');
             this.connection.disco.addFeature('urn:xmpp:jingle:transports:ice-udp:1');
+            this.connection.disco.addFeature('urn:xmpp:jingle:transports:dtls-sctp:1');
             this.connection.disco.addFeature('urn:xmpp:jingle:apps:rtp:audio');
             this.connection.disco.addFeature('urn:xmpp:jingle:apps:rtp:video');