Remove several using namespace std

 - They cause compilation failure (on OS X 10.9 with clang defaulting 
   to C++11) 
 - Failure occurs because unique_lock and friends is both in namespace 
   std and namespace boost.
Merge pull request #22 from NikolausDemmel/hydro-devel

Fix compilation on OS X 10.9 (or anywhere else with a C++11 compiler)