witnesses, registering witness url.
Derive memo, witness, etc. keys from the active key.
Make witness_create_operation accept relative key identifiers.
Prevent wif_to_key from throwing on invalid base58 input.
Make witness_node accept witness keys in WIF format.
Re-enable shutdown in node code which was causing some (not all) of the
errors in two_node_network test #55, and likely occasional crashes at
shutdown for the witness node
