Add a real test suite

[amotl]

Hi there,

currently, this program completely lacks a test harness. In the light of a few issues recently reported here, it would be good to have one. Bringing this in could probably be done together with GH-44.

With kind regards,
Andreas.

/cc @MauritsDescamps, @ohobby

References

• ImportError: cannot import name 'soft_unicode' from 'markupsafe' #33
• ImportError: cannot import name '_literal_as_text' from 'sqlalchemy.sql.expression' #34
• UnicodeEncodeError when emitting station data to PostGIS #42
• KeyError: 'station_id\\' when exporting stations to file #48