This adds a epub2mobi conversion rule by use of ebook-convert

It is still a very rough conversion and there are loads of
additional tweaking options available.
(see `ebook-convert foo.epub foo.mobi -h` to get a list
of them displayed).

`ebook-convert` is provided by the calibre package which
has also been added to the requirements.
Merge pull request #161 from dietmarw/mobi-version

This adds a epub2mobi conversion rule by use of ebook-convert