Fixes epubcheck errors: Mimetype not first file in archive
Fixes epubcheck errors: Mimetype not first file in archive

In order to be a completely valid EPub the mimetype file must be first
in the archive and be stored uncompressed with no extra file data.

- Switched from zipruby to rubyzip. Nothing I tried with zipruby allowed
  me to write the mimetype data uncompressed in a clean way.
- Had to use a Tempfile for the buffered output since rubyzip appears to
  only work with files.

jugyo/eeepub,YesVideo/eeepub