Change Financial Type for a contribution : insert reversing financial_trxn rows to insure all data is included in exports

Previous spec for this was to only insert financial_item rows when changing FT for a contribution. The problem w/ that approach is that all batch assignment and exports are based on financial_trxn rows. So any change which occurs AFTER a contribution is exported AND without creation of a financial_trxn row will not be included in subsequent exports.

I've updated the spec here:
http://wiki.civicrm.org/confluence/display/CRM/CiviAccounts+4.3+Data+Flow#CiviAccounts4.3DataFlow-Changecontributionfinancialtype%28backofficeform%29

So if we are inserting financial_items due to change in financial_account, then we also need to insert reversing financial_trxn rows.