Allow storage nodes to publish an arbitrary text description

[mhazinsk]

It would be useful for storage nodes to have a text field longer than the node nickname to store optional machine-readable descriptive information (e.g. physical location, owner information, etc). Once #467 is implemented, this could assist clients in choosing which storage nodes to use for uploads.

One potential use case of this is client-side mitigation of a Sybil attack if the introducer FURL string gets compromised. Clients could, for instance, refuse to use a storage node for uploads unless it can produce a PGP-signed message linking its node ID to a key in the client's local trust db.