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/main/Main.cpp

@@ -143,6 +143,9 @@ void printHelp (const po::options_description& desc)
 #endif // RIPPLE_ENABLE_MULTI_SIGN
            "     stop\n"
            "     submit\n"
+#if RIPPLE_ENABLE_MULTI_SIGN
+           "     submit_multisigned\n"
+#endif // RIPPLE_ENABLE_MULTI_SIGN
            "     tx <id>\n"
            "     unl_add <domain>|<public> [<comment>]\n"
            "     unl_delete <domain>|<public_key>\n"