[boost] [nowide] Add C++ 11 support

This is a library I'd like to see in Boost proper. I plan to use it in a current project. One enhancement that would make it easier to work with is supporting new C++11 interfaces for the adapters it implements. Things like explicit fstream (const string& filename, ios_base::openmode mode = ios_base::in | ios_base::out); Any plans to do that? I'm happy to do the work and share it on Github.