Fix handling of datetimes

[simleo]

Fixes #36

• On-the-fly conversion between strings and datetimes is now performed only when accessing the datePublished property of an entity. This allows the Python user to get and set it as a datetime object if desired, but now without causing problems with serialization
• The above conversion now also works for Python < 3.7
• datePublished is now always written as ISO 8601
• The data structure returned by crate.metadata.generate() is now JSON-serializable out-of-the box

Se the issue for more details