Proof-of-capacity whitepaper

Perhaps this should be moved to NebulousLabs/Experimental? Or maybe we should just have a Papers repository...
I wouldn't phrase it in a way that makes poc sound superior or inferior to pow, I would just talk about some of the values and tradeoffs.

The original reason that we wanted poc was just to have a host be able to announce a certain volume of free storage and have the network be reasonably sure that the storage actually existed.

So, at least in the introduction, I'd focus less on what it could be used for and more on what it is.