add PHP-FineDiff by @gorhill as diff engine
add API wrapper method to get item package file
add a first version of item comparison

Add a page that compares two items and outputs
a visual diff. There are still some tasks to do,
but this is basically the structure.
add error skeleton to compare page
add proper error handling to comparison page
add missing navigation and footer to comparison page
candidates: link to diff

In an open candidate, where a previous version of the item
is already in the stdlib, do now link to a diff page of the
candidate and the old version.
add compare link to review page
fix review diff links

For items with no older reviewed items, an error occured.
add a small icon for diffs
link to comparison from item view page
some minor styling improvements on compare page
begin implementing metadata diff

Provide a special diff view for metadata changes.
However, all properties returned as arrays from
the API are not yet included.
remove schema file from file-comparison

Everything relevant should be covered by metadata comparison.
better styling for diff page
diff page: redirect to correct URL

If version numbers in wrong order, do automatically redirect.
Merge pull request #20 from maul-esel/visual-diff

Add a visual diff for two versions of the same item. Especially useful for reviews and candidates.