IPFS block store

[greglook]

This should actually be in a separate repo (blocks-ipfs?), but there's a fairly straightforward mapping from the BlockStore protocol to the IPFS API.

• stat maps to /block/stat
• list is a bit of a question mark, since it doesn't make sense to try to enumerate every block in the IPFS network.
• get maps to /block/get
• put! maps to /block/put
• delete! has no equivalent, and should throw an exception.

IPFS would be pretty useful as a fallback lookup store, so that public IPFS data could be referenced transparently from blocks in a private store.

[greglook]

If implemented, this should be a separate library.