Handling folders in File API

[m-mohr]

The API specification doesn't explicitly specify whether it is possible to delete folders or just files. We may want to allow deleting folders. Also, other endpoints need to be more concrete how to handle folders. Putting folders may not be possible though. Issue may be obsolete with #135.

[m-mohr]

Added the following descriptions for folder handling:

• GET /files/{user_id}: Folders MUST NOT be listed separately so each element in the list MUST be a downloadable file.
• GET /files/{user_id}/{path}: If a folder is specified as path a FileOperationUnsupported error MUST be sent as response.
• DELETE /files/{user_id}/{path}: Resulting empty folders MUST be deleted automatically. Back-ends MAY support deleting folders including its files and subfolders. If not supported by the back-end a FileOperationUnsupported error MUST be sent as response.
• PUT /files/{user_id}/{path}: Folders are created once required by a file upload. Empty folders can't be created.