Introduce egeloen/http-adapter (road to PSR-7)

This PR is a proposal for #344. 

I have removed the previous adapters but maybe we should keep and deprecate them. Let me know your feeling about it.
This `GeoIP2Adapter` is not a "real" HTTP adapter anyway. Moreover, the lib under the hood has changed a lot, and first stable version is `2.0`, which is not what we require right now. I suggest you to not focus on this adapter.

Introduce your lib into Geocoder, and leave the geoip2 adapter alone. I will take care of it.