added a timer abstraction and replaced the use of boost.date_time. the timers used now are QueryPerformanceCounter on windows, mach_absolute_time on darwin and currently falls back on boost.date_time on other platforms. This has the advantage of being a monotonic clock, and also slightly more efficient

test/test_upnp.cpp

@@ -1,10 +1,8 @@
 #include "libtorrent/upnp.hpp"
 #include "libtorrent/socket.hpp"
-#include <boost/date_time/posix_time/posix_time.hpp>
 #include <boost/bind.hpp>
 #include <boost/ref.hpp>
 
-using boost::posix_time::seconds;
 using namespace libtorrent;
 
 void callback(int tcp, int udp, std::string const& err)