improve C-c C-a (ledger-xact-insert-transaction) behaviour

When the right place to add a transaction is after all existing
transactions, add it just after the last transaction rather than at the
end of the buffer.  (Otherwise the transactions get added after Local
Variables blocks and any other endmatter.)
Merge pull request #279 from csrhodes/fix-ledger-xact-add-transaction

improve C-c C-a (ledger-xact-insert-transaction) behaviour