Add Multi-signers information to account_info [RIPD-479]

It is convenient to be able to find out whether an account is
multi-signing and who those signers are without digging through
ledger history.  This commit adds fields to the data returned by
the account_info RPC command.  An account without multi-signers
returns data that looks like this (behavior is unchanged):

{
   "result" : {
      "account_data" : {
         "Account" : "rHb9C...tyTh",
         "Balance" : "99999987800000960",
         "Flags" : 0,
         "LedgerEntryType" : "AccountRoot",
         "OwnerCount" : 0,
         "PreviousTxnID" : "9DBC...99B7",
         "PreviousTxnLgrSeq" : 3,
         "Sequence" : 5,
         "index" : "2B6A...E9A8"
      },
      "ledger_current_index" : 3,
      "status" : "success",
      "validated" : false
   }
}

With this code a multi-signing account now returns data that
looks like this:

{
   "result" : {
      "account_data" : {
         "Account" : "rDg53...taMd",
         "Balance" : "10199998970",
         "Flags" : 0,
         "LedgerEntryType" : "AccountRoot",
         "OwnerCount" : 4,
         "PreviousTxnID" : "9DBC...99B7",
         "PreviousTxnLgrSeq" : 3,
         "Sequence" : 4,
         "index" : "3E85...8101",
         "multisigners" : {
            "SignerEntries" : [
               {
                  "SignerEntry" : {
                     "Account" : "rnUy2...rYEA",
                     "SignerWeight" : 4
                  }
               },
               {
                  "SignerEntry" : {
                     "Account" : "rPcNz...Wfux",
                     "SignerWeight" : 3
                  }
               }
            ],
            "SignerQuorum" : 7
         }
      },
      "ledger_current_index" : 3,
      "status" : "success",
      "validated" : false
   }
}