Un-opensource the repository

[ePaul]

Since Nakadi isn't open source for quite a while anymore (and therefore there likely is no usage of this library outside of Zalando), we should move the repository to an internal one (in the zalando-engineering-community organization).

I originally planned of doing this after most of the pending PRs are merged, but it seems like that won't happen anytime soon.

There might be a way of doing this while keeping the branches, PRs and issues intact (see our internal discussion in chat).

[ePaul]

In order to map the users, I'll need a list of all the users who have contributed here (with PRs and/or issues/comments), and map them to their internal user name (if existing).