Make comparison of extensions in zip addition case insensitive.
Make logic more concise.
Use format_plural(), 'cause it's awesome.
Get rid of explicit batch_process() call, and avoid dividing by zero...

... Don't run the batch if there are no files to add in the ZIP file.
Coding standards... squash some whitespace.
Merge pull request #100 from adam-vessey/7.x-case-sensitivity

Make extensions case insensitive, and a bit of misc cleanup.