Fix timezone related issues

This release corrects the use of timezones throughout the project. OpenWeatherMap returns dates in the UTC timezone. However, up until now, not all \DateTime instances returned by this API were using UTC as their timezone. This resulted in the problems described in #119, which were fixed in #120.
Thank you @Clpsplug for finding and reporting the issue as well as verifying the fix!

I really hope timezone handling is working as intended now. But timezone related code always seems prone to issues, so please open an issue if you encounter any problems with this release!

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