2-level transaction multi-signatures (RIPD-182):

This commit provides support for 2-level multi-signing of
transactions.  The ability is usually compiled out, since other
aspects of multi-signing are not yet complete.

Here are the missing parts:

 o Full support for Tickets in transactions.
 o Variable fees based on the number of signers,
 o Multiple SignerLists with access control flags on accounts,
 o Enable / disable operations based on access control flags,
 o Enable / disable all of multi-signing based on an amendment,
 o Integration tests, and
 o Documentation.

src/ripple/protocol/impl/TxFormats.cpp

@@ -112,6 +112,7 @@ void TxFormats::addCommonFields (Item& item)
         << SOElement(sfMemos,               SOE_OPTIONAL)
         << SOElement(sfSigningPubKey,       SOE_REQUIRED)
         << SOElement(sfTxnSignature,        SOE_OPTIONAL)
+        << SOElement(sfMultiSigners,        SOE_OPTIONAL) // submit_multisigned
         ;
 }