They are used to export a signed set of key images from a wallet
with a private spend key, so an auditor with the matching view key
may see which of those are spent, and which are not.
Signing is done using the spend key, since the view key may
be shared. This could be extended later, to let the user choose
which key (even a per tx key).
simplewallet's sign/verify API uses a file. The RPC uses a
string (simplewallet can't easily do strings since commands
receive a tokenized set of arguments).
