Add serialization with timezone to AwareDateTime

[theirix]

Allows AwareDateTime field to optionally serialize naive datetime with default timezone.
Behaviour is optional and enabled with use_serialization=True in ctor.

For example, if date-time is stored in a database without a timezone and we want to treat it as UTC datetime, new serializer helps a lot.

[odesenfans]

I was just looking for this change! I'm wondering: for some use cases, it makes sense IMO to localize aware datetimes as well (your fix just converts naive datetimes and leaves aware datetimes untouched). For example, using PostgreSQL + SQLAlchemy, datetimes are retrieved in the timezone of the DB connection (UTC by default).

It would be cool to ignore this implementation detail by just specifying my preferred output timezone in the schema and be done with it. What do you think?

[theirix]

Sorry, missed your comment. I think, your use case is better handled by creating a third DateTime descendant that stores a timezone passed in ctor and forces conversion an aware datetime (from DB) to this timezone for both serialization and deserialization. NaiveDateTime and AwareDateTime are a sort of mutually exclusive in handling awareness so it is not clear which class would handle proposed logic.