------------------
- Add basic serializers / deserializers
[laulaz]
- Display schedule viewlets only on views (not on folder_contents, cropping, etc.)
[laulaz]
- Fix error when validating an empty value for a schedule
[laulaz]
- Add missing key_type/value_type for Schedule field. This is needed (at least)
to export types schemas informations
[laulaz]
- Fix python3 compatibility : Use implementer instead of implements
[boulch]
- Handle multi schedules and exceptional closures
[fbruynbroeck]