Fix file format query param issue

Based on the surrounding code this *looks* like a bug, in that I think it will generate a url query fragment `&format=pngpng` rather than `&format=png`.
Dont include empty folder in built package.
Bump version. Update changelog.
Fix bug creating many preview buffers for non-persisted source
Always create and display temp-file.

This is a workaround for bug in OS X-version of Emacs. Addresses:
https://github.com/josteink/wsd-mode/issues/11
Merge branch 'master' of github.com:josteink/wsd-mode
Update changelog.
Fixup tests after changing behaviour.