HashTable: Added C++11 initializer_list constructor

e.g.
    HashTable<label, string> table1
    {
        {"kjhk", 10},
        {"kjhk2", 12}
    };

    HashTable<label, label, Hash<label>> table2
    {
        {3, 10},
        {5, 12},
        {7, 16}
    };