[thread] lockable traits and concepts classes adding proposal

Hello boosters ,

While writtig a generic library I have needed some lockable traits 
and concepts to be chacked with BCCL.

If you think that these classes are of general use it will be a pleasure for 
me, if Anthony accepts, to included them on the Boost.Thread library. 
Otherwise I will left them in my Sync library. For the moment all is defined 
in a sync (as synchronization) namespace and directory but this is without 
importance.

Attached a short introduction (in quickbook format- sorry I can not attach the html file) as well as the
header files (which should with gcc 3.4 on cygwin)

Of course all this is subject to discusion, naming and file structure 
changes, ...

For each one of the mutex in the thread library, we need to
* make the mutex inherit from the helper lock_traits_base or
* have nested typedefs or
* specialize each one of the traits templates

As an external library I have do this in a non-intrusive way as for example:

template<>
struct timed_interface_tag<boost::mutex> {
    typedef hasnt_timed_interface_tag type;
};
template<>
struct reentrancy_tag<boost::mutex> {