Add this_thread namespace

[chusitoo]

First and foremost, thank you for this gread lib. In the current project, I have to make a call to this_thread::sleep_for() in parts of the implementation (using a separate thread for endpoint). In order to respect the current design that makes a distinction between boost and stl, I've added a quick patch in my code that looks something like this:

#ifndef _WEBSOCKETPP_CPP11_STL_
#include <boost/thread/thread.hpp>
#endif

...

namespace websocketpp {
namespace lib {
#ifdef _WEBSOCKETPP_CPP11_STL_
namespace this_thread = std::this_thread;
#else
namespace this_thread = boost::this_thread;
#endif
} // namespace lib
} // namespace websocketpp

...

websocketpp::lib::this_thread::sleep_for(websocketpp::lib::chrono::milliseconds(1000));

Ideally, I'd see this as code that thread.hpp would take care of, which is exactly the goal of this PR.