Improve naive/aware DateTime objects and documentation

[jenstroeger]

I think it would help to clarify in the documentation that a deserialized datetime object is by default aware (i.e. its tzinfo attribute is set to a timezone object provided by the pytz non-standard library) rather than being naive (i.e. its tzinfo attribute is None).

In this context, normalizing while deserializing an aware datetime into a naive datetime would be useful for those applications which operate purely on naive datetime objects. For example, deserializing a string with +00:00 as a timezone might simply strip tzinfo, whereas other offsets ought to be added to a naive datetime using a timedelta derived from the offset.

[tseaver]

@jenstroeger Hmm, the current docs seem to me to make it clear that naive datetimes get a timezone applied using the default_tzinfo attribute. Users who want to suppress that conversion need to pass tz_info=None when constructing the schema node / type. E.g.:

import colander

class Example(colander.Schema):
    naive_stamp = colander.SchemaNode(
        colander.DateTime(default_tzinfo=None),
    )

Please feel free to reopen if I've missed the point here.