Proof-of-capacity whitepaper

Perhaps this should be moved to NebulousLabs/Experimental? Or maybe we should just have a Papers repository...
We should really write a paper or find a paper (find would probably be better) that explains how to do random numbers in a consensus system.

There are several approaches to generating random numbers and they each have vulnerabilities but there's at least one method that's pretty strong (progress free POW followed by sequential POW).

That doesn't need to be discussed in this paper but we should reference at least one source, if not several, that explain where the random number could potentially come from.