The original idea was to resolve #118, which wants an easy feature to have all datetime.datetime fields as UTC-aware values.

This is definitely something very, very useful, as many people (including me) consider naive datetimes a source of problems.

The proposed solution in #118 is more field-specific, but considering the issue title and the fact that most people either want timezone awareness or not (everywhere), I feel like this is the best way of getting there.

The solution here tries to:

• be simple to apply: generally, if someone is interested in timezone awareness, they will want this for all cases, so no need to configure it one by one (or maybe by document class).
• not break current API: all projects already working should still work normally.
• allow certain flexibility: through the CodecOptions object, we are able to not only define that timezone-aware datetimes is what we want, but also other conversion stuff that some people may find useful.

I must say that I believe that the solution could be cleaner if the AIOEngine and SyncEngine classes were not obfuscating the self.database object generation. I was thinking of allowing an input of a full database object, but technically it stops making sense to input a client, so I had to drop the idea, as having both client and database as input arguments stops making sense (the database contains a reference to the client). It would be dirtier, and a possible misuse could happen.

So if there are plans for breaking the API at a certain point, it should be considered to change the initialization of these engines, extracting the database object generation out of the BaseEngine subclasses, to facilitate custom configurations.

Close #118