Added support for Slim 3

[KeizerDev]

I tested the Container class as that was the thing I'm using this library the most for, the Config class isn't working yet as static::$slim->config($key) will result in a method not found.

[KeizerDev]

Everything works now except for Config. I'm not sure what it was exactly but it seems not to be there in Slim anymore as they only use Containers now. Should I remove it?

[KeizerDev]

Is this going to be merged anytime soon? As I need it for my projects I would like to have it added as soon as possible! If you don't have time for maintothis package I'll probably make a fork and release it that way.

[johnstevenson]

Thanks for your work on this. I originally expected that when Slim 3 was released there might be a need to update this repo, but no one ever asked! I'm not sure how long Slim 3 will be supported now that Slim 4 is a thing, so let me know if you want to fork this repo (and I'll deprecate this package).

[KeizerDev]

I came to the need of this inside my own projects (which are still built with Slim 3) and because of the delay of responding and the need for me to have this I forked it and submitted my fork to composer.
I'm not sure if I will update my fork to support Slim 4 but hey it still got Slim 3 support for now, the fork can you found here: https://github.com/KeizerDev/slim-static