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/miniupnpcstrings.h

@@ -0,0 +1,27 @@
+/* $Id: miniupnpcstrings.h.in,v 1.5 2012/10/16 16:48:26 nanard Exp $ */
+/* Project: miniupnp
+ * http://miniupnp.free.fr/ or http://miniupnp.tuxfamily.org/
+ * Author: Thomas Bernard
+ * Copyright (c) 2005-2011 Thomas Bernard
+ * This software is subjects to the conditions detailed
+ * in the LICENCE file provided within this distribution */
+#ifndef MINIUPNPCSTRINGS_H_INCLUDED
+#define MINIUPNPCSTRINGS_H_INCLUDED
+
+#if   defined(_WIN32)
+#define OS_STRING "Windows"
+#elif defined(__linux__)
+#define OS_STRING "Linux"
+#elif defined(__OSX__)
+#define OS_STRING "Mac OS X"
+#elif defined(__APPLE__)
+#define OS_STRING "Mac OS X"
+#elif defined(__DARWIN__)
+#define OS_STRING "Darwin"
+#else
+#define OS_STRING "Generic"
+#endif
+#define MINIUPNPC_VERSION_STRING "1.7"
+
+#endif
+