Support for transforms in fieldnames

[aodin]

In order to support transforms in fieldnames (e.g. datetime__year), I added AttributeError to the caught expections of io.to_fields. This changes the try block to match what is found in the Django ORM source, which prevents the AttributeError that would be raised by to_fields attempting to parse year as a related field instead of a transform.

Improper transforms or non-existent relations will still raise django.core.exceptions.FieldError during the try_transform of query processing in the Django ORM.

[coveralls]

Coverage decreased (-6.7%) to 83.537% when pulling 8acb337 on aodin:master into 8276d69 on chrisdev:master.

[coveralls]

Coverage remained the same at 90.244% when pulling 9b27ec1 on aodin:master into 8276d69 on chrisdev:master.

[aodin]

Apparently Django's handling of transforms is heavily dependent on version. Closing for now until I have a better understanding of Django's ORM internals.