support of multiple account identifier summaries

See the * note at the bottom of section 3.03 of the BAI spec

It reads:
  "* Type 03 records may report several different status and/or summary
  amounts for the same account. For example, a single 03 record might
  report ledger balance and available balance, as well as the amount, item
  count and funds type for total credits and total debits. The “Type
  Code,” “Amount,” “Item Count” and “Funds Type” fields are repeated to
  identify each status or summary type. See Appendix A for Type Codes."

This changes the structure that we return for AccountIdentifier records.
Now we'll have a :summaries entry in the map, that contains a submap for
each account summary.