Proof of Storage

I'm not 100% sure that this works, because the testing is incomplete, but I think that this is all that remained for doing proof of storage. It adds a function to wallet tree for finding the n'th atom in a quorum, it adds proof of storage to `newHeartbeat()` and it adds proof of storage to the verification - though if a host fails proof of storage all that happens is a message is output, because auto-repair isn't enabled yet and so new hosts will continuously fail their storage proofs.

So what remains is adding in auto repair (tonight), then doing a general code overview (tomorrow), then creating a dev branch and setting up a quorum that's something like 16 nodes big, spanning multiple computers, multiple connections, including the kimsufi.

We'll test that internally through Friday, and then if we're happy with how the tests go we'll prepare a launch.
`PoStorage` is a weird abbreviation. Should probably be either `PoS` or `ProofOfStorage`. Or maybe `StorageProof`. Or maybe just `Proof`. Just not `PoStorage`.