Kinto-attachment

**New feature**
- Expose the API capability `attachments` in the root URL (35)

**Internal changes**
- Upgrade tests for Kinto 1.11.0 (36)

**New feature**
- Setting to store files into folders by bucket or collection (fixes 22)

**Bug fixes**
- Remove existing file when attachment is replaced (fixes 28)

**Documentation**
- The demo is now fully online, since the Mozilla demo server has this plugin
installed.
- Add some minimal information for production