Added station id retrieval at startup

Added station id retrieval in StationStore at object initialization.
A map is built with the ids as keys and the stations a values.

As a result, the id argument in StationStore.getStation is tested against the map.
If it is not present in the map, the function throws an
IllegalArgumentException.
FIX: Fixed response handling in getStation + cleanup

Fixed response handling in getStation where the response was sometimes transformed with some unitialized bytes into a string.
This would make the parsing function crash as it tried to interpret a
bad XML document.