Fix api changes

Changes in the MVG api:

The id in Station objects was changed. Before, it was an int, now it is a string with the format de:09162:xxxx where xxxx is the old id.

Changes in the library

As a consequence, get_stations() and get_nearby_stations() now return these new IDs. get_departures() and get_route() now accept both the new string IDs and the old integer IDs to keep backwards compatibility.

Thanks to @Bastlwastl42 for their pull request :)
closes #15.