A bank had an offset on the date of the transactions...
It also passes the interal Bank type to the converter, to access it's
additional options before converting.
The period will only apply after the transactions have been received,
since if sent on the request, credit cards seem to use the value date
instead of the booking date, rendering the feature useless.
Also adds some robustness to the download of the transactions, by adding
a retry in case of timeout.
There is now the possibility to download the transactions for all banks
in the banks table in the DB.
`NordigenInput` parse method fully functional, and entire chain from
downloading to parsing (simple w/ converter) to writing to DB.
DbTransaction type added __conform__ to simplify writes to DB.
Get bank methods added to both `Manager` and `DatabaseClient`.
Warning: csv parser most likely not working at this point.
Issues #16 and #17
