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.

I'm afraid that the exact reasons that this resolves the C++11 ambiguity aren't quite clear to me, so I hope this is a valid standards compliant solution.  It does fix the compile error with g++-4.6 noted in issue #13 and is fairly non-invasive.