Air pollution implementation

[Samo8]

Hi, I needed Air pollution functionality for my personal needs. I found package implementing OpenWeatherMap, but without Air pollution, so I decided to add this functionality.

[tanzilzubair]

Hello!
Firstly, I would like to thank you for your interest in the project! Secondly, I would like to say that your contribution to the project is greatly appreciated.

I had wanted to update this project myself, with air pollution as well as a few more of the end-points (but it looks like you beat me to it!), but alas, I am currently very busy with some things, and as such, cannot spare the time to even review this pull request properly, much less write updates for the project myself.

As such, I would like to kindly request some patience from you. This pull request will remain open, and at the earliest I can spare sufficicent time, I will review and merge it, or get back to you regarding changes.

Again, thank you very much for your help and contributions!