Added hash implementation for std::pair (from the web)

[dan1994]

Closes #29

[Roynecro97]

I know this is already merged, but the hash is very weak, all pairs with first == second will have a hash of 0. I think that returning the hash on the concatenation of the hashes on the members of the pair might be better. (Of course it will not be better performance wise)

[dan1994]

I actually noticed that, when I went over it later, and your comment is correct of course.
I'll open an issue to fix that (correctness before performance).