Add unit tests for wallet keypair generation:

* Allow `passphrase` to be a seed encoded in any of three formats or a
    literal passphrase.
  * Recognize the absence of `passphrase` as requesting a random seed.

Extract walletPropose() and keypairForSignature() as separately factored
functions (from doWalletPropose() and transactionSign() respectively) to
facilitate unit testing.

src/ripple/rpc/impl/KeypairForSignature.cpp

@@ -0,0 +1,55 @@
+//------------------------------------------------------------------------------
+/*
+    This file is part of rippled: https://github.com/ripple/rippled
+    Copyright (c) 2012-2015 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/KeypairForSignature.h>
+
+namespace ripple {
+namespace RPC {
+
+KeyPair keypairForSignature (Json::Value const& params, Json::Value& error)
+{
+    if (! params.isMember ("secret"))
+    {
+        error = RPC::missing_field_error ("secret");
+
+        return KeyPair();
+    }
+
+    RippleAddress seed;
+
+    if (! seed.setSeedGeneric (params["secret"].asString ()))
+    {
+        error = RPC::make_error (rpcBAD_SEED,
+            RPC::invalid_field_message ("secret"));
+
+        return KeyPair();
+    }
+
+    KeyPair result;
+
+    RippleAddress generator = RippleAddress::createGeneratorPublic (seed);
+    result.secretKey.setAccountPrivate (generator, seed, 0);
+    result.publicKey.setAccountPublic (generator, 0);
+
+    return result;
+}
+
+} // RPC
+} // ripple