SigningAccount not serializable to transaction [RIPD-480]

Changed the implementation of get_signingaccount so the returned
SigningAccount object is build using JSON only.  This allows us
to change SigningAccounts so it is no longer serializable in a
transaction.

While I was in there, I also changed the SigningAccounts encoding
so it is treated as uncommon.

The change also had the side effect of relocating where the
SigningAccount field is returned, probably for the better.

// Old version
{
   "result" : {
      "status" : "success",
      "tx_blob" : "1200...61E1",
      "tx_json" : {
         "Account" : "rDg53Haik2475DJx8bjMDSDPj4VX7htaMd",
         "Amount" : "1000",
         "Destination" : "rHb9CJAWyB4rj91VRWn96DkukG4bwdtyTh",
         "Fee" : "10",
         "Flags" : 2147483648,
         "Sequence" : 4,
         "SigningAccount" : {
            "Account" : "rPcNzota6B8YBokhYtcTNqQVCngtbnWfux",
            "MultiSignature" : "3043...4F41",
            "PublicKey" : "02FE...01F8"
         },
         "SigningPubKey" : "",
         "TransactionType" : "Payment",
         "hash" : "97011...9065E"
      }
   }
}

// New version
{
   "result" : {
      "SigningAccount" : {
         "Account" : "rPcNzota6B8YBokhYtcTNqQVCngtbnWfux",
         "MultiSignature" : "3044...9565",
         "PublicKey" : "02FE...01F8"
      },
      "status" : "success",
      "tx_blob" : "1200...37E8",
      "tx_json" : {
         "Account" : "rDg53Haik2475DJx8bjMDSDPj4VX7htaMd",
         "Amount" : "1000",
         "Destination" : "rHb9CJAWyB4rj91VRWn96DkukG4bwdtyTh",
         "Fee" : "10",
         "Flags" : 2147483648,
         "Sequence" : 2,
         "SigningPubKey" : "",
         "TransactionType" : "Payment",
         "hash" : "A660...B46B"
      }
   }
}