add owm.py: get weather data from openweathermap

this script can replace wetter.at.pl:

perl:   2018-06-08;19:00;   05:21   ;21:09   ;21   ;0 mm/h;7km/h O     ;stark bewölkt      ;93
python: 2018-06-08;20:20:00;05:20:24;21:10:53;17.13;      ;2.6km/h O   ;Überwiegend bewölkt;75

and can be extended to write to the mysql database and a csv file like
seewasser.py

(and probably parts of seewasser.py could be extracted as common functions)

owm.py needs an api key from openweathermap and some config; cf. the
comments ath the top

questions:
- is precipitation really missing from json and available in xml?
- the str() casting at the end looks ugly