Add get_multisignature command [RIPD-479]
Add submit_multisigned command [RIPD-480]
Functional multi-signature validation [RIPD-480]

This implementation is proof of concept only.  The validation
is in the wrong place.  Future commits will move the signature
validation into the transaction engine.
Change get_multisignature to get_signerentry [RIPD-480]

We learned that a SignerEntry requires three fields:
 - an AccountID
 - a public signature for that AccountID
 - the signature on the multi-signed transaction.
So we changed get_multisignature to return all three fields
as a SignerEntry object.  We changed the name as well, since
we're now returning more than just a MultiSignature.