Update Weather forecast to mention and use Dark Sky

[alni]

Need to update the Weather forecast to use and mention Dark Sky instead of Forecast IO.

• [x] Rename FORECASTIO AND FORECAST_IO variables (and others with similar names) to DARK_SKY etc.
• [x] Change API URL to the correct one
• [x] Change Powered by Forecast to the correct attribution needed now by Dark Sky
• [x] Update the python-forecastio library to the newest
• [ ] Add support for all current supported languages of Dark Sky with the summaries
• [x] Update the README to mention Dark Sky instead of Forecast IO
• [x] Update code comments and strings to mention Dark Sky instead of Forecast IO