Add Rails Migration Generator

Boths of these indexes are in the original implementation:  https://github.com/vitaly/debitcredit/blob/master/db/migrate/20140128131859_add_transaction_parent.rb#L4 and https://github.com/vitaly/debitcredit/blob/master/db/migrate/20140121181304_create_debitcredit_transactions.rb#L11

I just copypasted.