Allow filenames for path parameter

[LukasLohoff]

Right now, the loader accepts only folder names for the "path" parameter. For a webdav folder with multiple zip files, it's useful to also support filenames.

For example:

curl -d "share_url=https://uni-muenster.sciebo.de/index.php/s/G8vxQ1h50V4HpuA"  \
    -d "path=/metatainer.zip" \
     http://…/api/v1/compendium

[nuest]

And it would be good to make clear if that requires a leading / or not :-).

[LukasLohoff]

Specifying filenames works now, the file has to end in .zip, otherwise it can't be detected.

A leading slash is now added if necessary, so both options should work.

See #42

[nuest]

Great. Please also create a PR for the API docs.
Will close this after merging the new code.