If for some reason there is not conversion raise an exception.

Otherwise the caller receives a None and some weird error can appear
at random location.
Merge pull request #16 from gipi/graceful-exception

If for some reason there is not conversion for a field raise an exception.
Cleaning up orm
Misc cleanups and support for hidden fields w/empty value.
Do not access the namedtuple when setting up fields
Support for nullable date/time fields
Version bump for bugfixes.
Adding support for `BigIntegerField`, fixes coleifer/flask-peewee#74
Fixing none values in hidden query field.
Use six to be Python 3 compatible
Listify this map since code will expect a list
Require six when installed
Removed six dependency in favor of a compatibility module.
Merge pull request #17 from duncaningram/python3

Make python 3 compatible
choices field could be callable