M of n entries

Intermediate state of the multi-signed transaction code.  The following have been added and work correctly to the best of my knowledge:

 o SignerListSet adds (or removes) multi-signers on an account.
 o get_signingaccount requests a SigningAccount to be used when multi-signing a transaction.
 o submit_multisigned submits a multi-signed transaction to rippled for processing.

No work has yet been done with fees and reserves.  There is plenty of work remaining.  This is just the bare bones.

This is a bogus pull request that will make it easy for people to comment on the code without the risk that it might accidentally be merged to upstream/develop.