Fix irsa_dust to handle new format of dust service responses

Fix for #241. Recently the IRSA dust service changed the format of its xml responses to include more data in the extinction/statistics section, which the irsa_dust module couldn't handle. This pull request updates irsa_dust to read the new format. It also updates the irsa_dust/tests/data files so they reflect the new format (and removes besancon_test.txt from the irsa_dust/tests/data directory, since it didn't belong there).