Change del.p_=0 to del.p_=nullptr (#22)

* Change del.p_=0 to del.p_=nullptr

A simple change (I hope) that cleans up compiler warnings and errors.

* Support older compilers

* Update throw_exception.hpp

* Update throw_exception.hpp

include/boost/throw_exception.hpp

@@ -116,7 +116,7 @@ public:
 
         boost::exception_detail::copy_boost_exception( p, this );
 
-        del.p_ = 0;
+        del.p_ = BOOST_NULLPTR;
         return p;
     }