WIP for distribution changes

[ghost]

@tomschr Can you take a look here, please?

The original goal here was just to make sure the bookmarklet is shipped again. And then I noticed that my tarballs were all horribly bloated with stuff that is not fit to ship.

I am a bit unsure about some of these changes:

• Why does Travis insist I mention all those dot files (travis, cookiecutterrc, bumpversion, landscape, ...) that nobody wants?
• Why do I need to include so many things that were never explicitly added and don't contain any Python files (e.g. experimental-checks)?
• Can I check whether the manifest file is ok somehow? (= test)

I already see that Travis has issues... blargh.

[tomschr]

Discussed and resolved with Stefan all the open questions.