Firefox: new method for request handling

Now both nsIContentPolicy and on-http-* observers are used for net request
monitoring.

Reasons:
 - In many cases, nsIContentPolicy.shouldLoad is invoked twice for the same
   resource, because of the speculative parsing.
 - nsIContentPolicy.shouldLoad don't have information about the channel,
   so it can't redirect the request, nor change its headers, however
   on-http-opening-request can.

Also, local mirroring and inline-script blocking has been implemented.

platform/firefox/vapi-client.js

@@ -108,12 +108,14 @@ vAPI.messaging = {
     },
 
     close: function() {
-        if (this.connector) {
+        if ( !this.connector ) {
+            return;
+        }
+
         removeMessageListener(this.connectorId, this.connector);
         this.connector = null;
         this.channels = {};
         this.listeners = {};
-        }
     },
 
     channel: function(channelName, callback) {