Update data and generalize data file finder

Providing new Bundesbank data (valid 2014/06/09 til 2014/09/07).

I also refactored the `DATA_FILE` finder. Instead of manually checking all the possible dates, a simple map/find will do the trick. In future (i.e. until the SEPA introduction is finally finished), we only need to provide new `yyyy_mm_dd.tsv.gz` files, the rest happens automagically.