New feature: retrieve weather currently measured by a meteostation

[csparpa]

As read on the OWM API wiki:

Get the most recent measurements from weather station, ID = STATION_ID
Data can be called in JSON or HTML.

Example: http://api.openweathermap.org/data/2.5/station?id=1000

{"station":{"name":"KPPQ","type":1,"status":50,"id":1000,"coord":{"lon":-90.47,"lat":39.38}},"last":{"main":{"temp":279.15,"pressure":1022,"humidity":87},"wind":{"speed":2.1,"deg":230},"visibility":{"distance":16093,"prefix":0},"calc":{"dewpoint":277.15,"humidex":278.15},"clouds":[{"condition":"CLR"}],"dt":1415187300},"params":["temp","pressure","humidity","wind","visibility"]}

We need to introduce at least:

• owm25.weather_at_station(id)
• a new specific parser StationObservationParser, as the JSON is quite different from the one returned when querying for weather on city IDs. Next step then could be to refactor commonalities between the 2 parsers

[csparpa]

Implemented with PR #62