Support encrypted transport, properly handle multiple recipients.

1) Add encryption support for the transport layer.  This obscures
   metadata from the push messaging provider.

2) Better support the direction multiple destination messages is
   headed (one unique message per recipient).

src/org/thoughtcrime/securesms/util/TextSecurePreferences.java

@@ -59,6 +59,10 @@ public class TextSecurePreferences {
     setStringPreference(context, SIGNALING_KEY_PREF, signalingKey);
   }
 
+  public static String getSignalingKey(Context context) {
+    return getStringPreference(context, SIGNALING_KEY_PREF, null);
+  }
+
   public static boolean isEnterImeKeyEnabled(Context context) {
     return getBooleanPreference(context, ENTER_PRESENT_PREF, false);
   }