Make nested boost::assign::map_list_of work in C++11 mode

Adding a layer of indirection here seems necessary to avoid ambiguities
introduced by new C++11 features.  The extra layer forces a call to the
implicit conversion operator defined for converting the boost
initialization list format to std::map.
Merge pull request #14 from c42f/assign-cxx11-fix

Make nested boost::assign::map_list_of work in C++11 mode