Add support for hashmaps (wrap std::unordered_map)

[msune]

As a user, I would like a to have a variant of cdada_map_t that is equivalent to std::unordered_map, which has the following differences:

	map	unordered_map
Ordering	ordered	unordered
Implementation	Red-Black Tree (self balancing)	Hash Table
Computational complexity
Search	log(N)	Average = O(1), Worst = O(N)
Insertion	log(N) + rebalancing	Average = O(1), Worst = O(N)
Deletion	log(n) + rebalancing	Average = O(1), Worst = O(N)