Handle current weather queries for multiple cities

[JesseEmond]

See http://openweathermap.org/current#cities for the documentation.

3 calls are needed:

• rectangle_zone, example call
• circle_zone, example call
• cities, example call

It appears to me that this would fit in the Current class, but the URLs are different (i.e. the URL for normal current weather differs from the one for rectangle/circle/cities). Maybe retrive could take an optional parameter to force a URL only for that request?

I will work on it and hopefully submit a PR in the following days.