Implement support and configuration for SSL

[csparpa]

As stated here, it is now possible to use SSL-based communications between OpenWeatherMap API and PyOWM

We need to:

• PyOWM can be configured to either use SSL or not (default should be: don't use it - as it might break existing PyOWM usage scenarios)
• change URI schemas accordingly to what config says (HTTP or HTTPS)

Check out for any potential issue with certification chain verification on the HTTP library/code that PyOWM uses to fetch data from the API

[csparpa]

Requests SSL Cert validation