OpenWeatherMapProvider: Fix API calls and responses

* The 16 day api which is currently called needs a paid-for account
* Use the free API w/ 5 days/3 hours forecast
  * The result-set was limited to 5 days anyway
* While reworking the API calls, remove unnecessary parameter "mode",
  which requests the API's default "json" anyway
* Modify the ForecastResponse class to be able to understand the new
  result
  * Remove class "Temp" - not used anymore
  * Introduce class "Main" which matches the new response's name and
    content
* Calculate the min/max temperatures for each day in the forecast manually
  as we get 5days à 3hours now
* Interpret wind speed correctly: result is m/s when requesting metric
  values but it's passed on (and interpreted) as kph, so multiply the
  speed w/ factor 3.6 in that case - imperial is already correct, no need
  to touch that

* Thanks to Justin Bouchard <jusking.bouchard@gmail.com> who found that
  the current API call isn't working properly

Change-Id: Ibdc3d3a3a4538fcc403dcb47c488baee8051d37e