Random.sample implemented

@subutai what do you think? This implementation may not be efficient when sampling a small number from a very large population but it has the potentially nice property of the selection being in the same order as the population.
Hmm, I'm not sure this is unbiased. If I understand it correctly, it will give preference to later elements in population. It may miss a bunch in the beginning, but the likelihood of selecting increases as you go down the list. Another simple approach is to randomly shuffle population and select the first num elements.