Add UPnP support.

Feedback is in logs as suggested by skid_au. The checkbox is still there, but
mostly for people who would like to opt out (unfortunately, I can not be sure
how this feature may behave for some routers - there's a hell of a lot of bad
UPnP implementations.)

The Visual Studio stuff is a little messy, so I apologize if anything is a bit
off. I tested most configurations and it worked.

I also tested CMake on Debian Wheezy, Ubuntu Saucy, and Mac OS X Mountain Lion.
All seemed to be OK.

Externals/miniupnpc/src/receivedata.h

@@ -0,0 +1,19 @@
+/* $Id: receivedata.h,v 1.4 2012/09/27 15:42:10 nanard Exp $ */
+/* Project: miniupnp
+ * http://miniupnp.free.fr/ or http://miniupnp.tuxfamily.org/
+ * Author: Thomas Bernard
+ * Copyright (c) 2011-2012 Thomas Bernard
+ * This software is subjects to the conditions detailed
+ * in the LICENCE file provided within this distribution */
+#ifndef RECEIVEDATA_H_INCLUDED
+#define RECEIVEDATA_H_INCLUDED
+
+/* Reads data from the specified socket.
+ * Returns the number of bytes read if successful, zero if no bytes were
+ * read or if we timed out. Returns negative if there was an error. */
+int receivedata(int socket,
+                char * data, int length,
+                int timeout, unsigned int * scope_id);
+
+#endif
+