Add a helper class that join a thread on destruction

[viboes]

Based on the thread_guard class defined in C++ Concurrency in Action define a helper class that join a thread on destruction.

// Based on the Anthony's idea of thread_guard in CCiA

namespace boost
{

  class thread_guard
  {
    thread& t;
  public:
    BOOST_THREAD_NO_COPYABLE( thread_guard )

    explicit thread_guard(thread& t_) :
      t(t_)
    {
    }
    ~thread_guard()
    {
      if (t.joinable())
      {
        t.join();
      }
    }
  };

}