C++11 allows list initialization for classes. STL map has been added
a constructor with the signature

    map(initializer_list<value_type> il,
        const allocator_type& alloc = allocator_type());

Added a similar constructor to bimap, allowing for initialization in the
form of initializer list, such as

    boost::bimap<double, long> tdn {
        {114.514, 1919},
        {891.931, 810}
    };