sign_for RPC command (RIPD-182):

The sign_for RPC command returns a field suitable for
signing a multi-signed transaction.  Actual multi-signed
transactions are in the next commit.

src/ripple/rpc/handlers/SignFor.cpp

@@ -0,0 +1,42 @@
+//------------------------------------------------------------------------------
+/*
+    This file is part of rippled: https://github.com/ripple/rippled
+    Copyright (c) 2012-2014 Ripple Labs Inc.
+
+    Permission to use, copy, modify, and/or distribute this software for any
+    purpose  with  or without fee is hereby granted, provided that the above
+    copyright notice and this permission notice appear in all copies.
+
+    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
+    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+    ANY  SPECIAL ,  DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
+    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+//==============================================================================
+
+#include <BeastConfig.h>
+#include <ripple/rpc/impl/TransactionSign.h>
+
+namespace ripple {
+
+// {
+//   tx_json: <object>,
+//   account: <signing account>
+//   secret: <secret of signing account>
+// }
+Json::Value doSignFor (RPC::Context& context)
+{
+    context.loadType = Resource::feeHighBurdenRPC;
+    NetworkOPs::FailHard const failType =
+        NetworkOPs::doFailHard (
+            context.params.isMember ("fail_hard")
+            && context.params["fail_hard"].asBool ());
+
+    return RPC::transactionSignFor (
+        context.params, failType, context.netOps, context.role);
+}
+
+} // ripple