Support for std::initializer_list in stable_vector

I miss support for std::initializer_list in some container methods, so I prepared patch that implements it for following method of stable vector (if the std::initializer_list is supported by the compiler):
* constructor
* operator=
* assign
* insert

I also added some simple tests, they pass with g++ and clang++.

If this patch will pass a review and be included in boost::container repository I will add similar support for other boost containers.