Move/refactor keepalive logic and add disconnect timer

We now disconnect ourselves if we don't get the server's response to a
keepalive request within 30s. This way we will eventually disconnect if
the network goes away but the socket is not closed.*

* See code.google.com/p/chromium/issues/detail?id=197841 and
https://stackoverflow.com/questions/11755605/chrome-websocket-connection-not-closed-when-browser-closed

We will then try to reconnect once a minute (See 8a10c96);

Keepalives belong at this level anyway, since the format is defined by
both the websocket resource protocol and our specific server url
structure.

// FREEBIE

libtextsecure/websocket-resources.js

@@ -51,6 +51,10 @@
             window.crypto.getRandomValues(bits);
             this.id = dcodeIO.Long.fromBits(bits[0], bits[1], true);
         }
+
+        if (this.body === undefined) {
+            this.body = null;
+        }
     };
 
     var IncomingWebSocketRequest = function(options) {