Proof-of-capacity whitepaper

Perhaps this should be moved to NebulousLabs/Experimental? Or maybe we should just have a Papers repository...
I'd like to see more evidence that this attack is feasible. If finding a typical hash requires `n` computations, then this attack should require at least `n^2` per hash. This could be significant if we require sufficient leading zeros as well.

Also, I don't think that this attack could reliably produce a normal distribution pattern. You'd probably wind up with two distinct clusters; a further-than-expected hump, and a closer-than-expected hump to make up for it. But I don't know how detectable this would be.