Fixed a minor error in one CSS file used for ePub files
(this minor error prevented the ePub from being validated)
(see #143)
Fixed the ePub toc.ncx template
Instead of the language of the book, the template was using
the book's author name as the ' xml:lang' attribute value

(this error prevented the ePub from being validated)
(see #143)
Replaced the Zip PHP extension by the OS zip command

The ePub format requires that the 'mimetype' file is
at the first file of the ZIP file and it's uncompressed.
The PHP Zip extension doesn't support compression levels,
so you cannot generate a valid ePub book with PHP.

The method now tries to use the OS 'zip' command if
available and then fallbacks to 'Zip' extension.

(this error prevented the ePub from being validated)
(see #143)
Updated the ePub test fixtures after the recent changes
(see #143)
Merge pull request #144 from javiereguiluz/fix_epub_errors

Fix epub validation errors