Add `std::initializer_list` constructor

[SuibianP]

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}
};