Module for generating slide image zip archives

The ZipSlides module supports generating a zip of all associated slides for any model that has the .slides array defined. For this to work we need to add the rubyzip gem and configuration for it in rubyzip.rb file.
include ZipSlides module

now presentation.zip_slides and master_group.zip_slides both return a blob with all the associated slides stored in a zip archive
Whitespace and comments

There were plenty of unnecessary blank lines and comments starting right after the # mark.
Remove no longer used @new_group from #index
Add slide download links

Now the user can download all the slides in the group as a zip.
Merge branch 'zip-slides-in-group'

Added functionality for downloading the full size images of all slides in a given group as a zip.
Add the new :download_slides to ACL test