net: Automatically create hidden service, listen on Tor

Starting with Tor version 0.2.7.1 it is possible, through Tor's control socket
API, to create and destroy 'ephemeral' hidden services programmatically.
https://stem.torproject.org/api/control.html#stem.control.Controller.create_ephemeral_hidden_service

This means that if Tor is running (and proper authorization is available),
bitcoin automatically creates a hidden service to listen on, without user
manual configuration. This will positively affect the number of available
.onion nodes.

- When the node is started, connect to Tor through control socket
- Send `ADD_ONION` command
- First time:
    - Make it create a hidden service key
    - Save the key in the data directory for later usage
- Make it redirect port 8333 to the local port 8333 (or whatever port we're listening on).
- Keep control socket connection open for as long node is running. The hidden service will
  (by default) automatically go away when the connection is closed.

src/torcontrol.h

@@ -0,0 +1,19 @@
+// Copyright (c) 2015 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+/**
+ * Functionality for communicating with Tor.
+ */
+#ifndef BITCOIN_TORCONTROL_H
+#define BITCOIN_TORCONTROL_H
+
+#include "scheduler.h"
+
+extern const std::string DEFAULT_TOR_CONTROL;
+static const bool DEFAULT_LISTEN_ONION = true;
+
+void StartTorControl(boost::thread_group& threadGroup, CScheduler& scheduler);
+void StopTorControl();
+
+#endif /* BITCOIN_TORCONTROL_H */