Advertises DTLS support in disco#info.

modules/xmpp/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:apps:dtls:0');
                 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');