Proof-of-capacity whitepaper

Perhaps this should be moved to NebulousLabs/Experimental? Or maybe we should just have a Papers repository...
At some point we should also talk about the expected distance between hashes, and what sorts of things follow from that.

On an exponential distribution, the expected value is what you would intuitively think, but the median is actually quite a bit higher than the expected, and the probability of being really far off is not terribly low. I think it's something like appearing to store 1/5th the hashes will happen 1% of the time to someone who is actually storing 100% of the hashes.