Refactor binder environment using nbgitpuller

[djhoese]

See https://discourse.jupyter.org/t/tip-embed-custom-github-content-in-a-binder-link-with-nbgitpuller/922

Basically the pytroll-examples would become the "content" repository and we'd have a separate one that acts as the "environment" repository. The README in pytroll-examples would only need to point to the right URL. The nice thing about this is that any new tutorials or changes to this repository won't require rebuilding the binderhub image; only changes to the environment repository will do that.