Describe template variables
Fix exif/raw_exif and add datetime documentation
Merge pull request #26 from matze/update-docs

Describe template variables
Merge pull request #27 from matze/misc-cleanup

Miscellaneous changes
Set `zip_gallery` only when one is available

This change introduces a new variable `zip_gallery` to the Jinja context. It is
only set to `settings.zip_gallery` if a directory has a zipped archive.
Add documentation for `zip_gallery`