Animal shelter

Not my idea, I've seen this one in `Cracking the code interview`. To put it simple, the shelter works as a queue and is filled with cats and dogs. If you want to adopt an animal, you will have two options:

- the oldest animal (any animal)
- choose the oldest cat or dog

I like it because you'll have to implement a queue-like data structure, it doesn't matter if you use arrays or linked lists (or whatever you want to list here).

If you like this exercise, I can create a PR (maybe in a week from now, no promises).