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/app/tx/impl/SetAccount.cpp

@@ -193,7 +193,7 @@ public:
         {
             if (!mSigMaster)
             {
-                m_journal.trace << "Can't use regular key to disable master key.";
+                m_journal.trace << "Must use master key to disable master key.";
                 return tecNEED_MASTER_KEY;
             }