[ADD] account_banking_nl_rabo_mt940

https://code.launchpad.net/~therp-nl/banking-addons/7.0_mt940_structured/+merge/225805
I'd suggest rewriting these two lines as

    self.current_transaction.message = '/'.join(sf for sf in subfields['REMI'] if sf)

If you'd rather go for `filter()` then you can replace the anonymous function with `None`, which is handled by `filter()` to do precisely this.