Make entropy-over-time graph

We have number-of-relays-over-time, and amount-of-bandwidth-over-time. We don't yet have entropy-of-consensus-weights-over-time.

The way to get it is to sum up all the weights in the consensus, then calculate, for each relay in the consensus, the probability it will be chosen (its weight divided by sum of weights) times - log_2 of that probability. Then sum up all those calculations, and you have the shannon entropy of that consensus. As an intuitive check, if the shannon entropy is b bits, that's equivalent to a Tor network with 2**b nodes that all have the same weights.

http://en.wikipedia.org/wiki/Entropy_%28information_theory%29 for plenty more background.

Damian said he'd be happy to use stem to help extract the numbers, so from Karsten's perspective hopefully this is just a graphing exercise.

Once we have this one going, I will likely ask for the same sort of graph as if we used descriptor bandwidths for weights (that is, as if we didn't have bwauths running), so we can compare how much entropy we're giving up in exchange for the performance improvements we get from bwauths.

And after that, I'll want to know about the subset of nodes that have the Guard flag, and the subset that have the Exit flag. One step at a time, I know, but I figure I should give a sense of the scope of the future plans too. Thanks!