fix std namespace for shared_ptr, weak_ptr, ...

I tried to use c++11 in an OF project but ofTypes.h redefines variable that are now part of the std namespace. This PR is a quick fix for that.

I only did a quick test and ofPtr still works. I wonder though if this is safe? is the std::shared_ptr of c++11 the same as the former std::tr1::shared_ptr?