Django-model-import

4358a9667f083d85301aea050d7dca882e54a012 Inlined import of `tablib`, so that it's not loaded on app startup, and so that it's not imported at all if the tablib parsers aren't used. Also renamed `ImportParser` to `BaseImportParser`; emphasising that it must be subclassed.

- [10] Improved date parsing logic in `DateTimeParserField` to better read "human" dates (ie. not big-endian). Also emphasised that little-endian is the default and provided a `middle_endian` flag for getting parser to use middle-endian form.

- Added JSONField support. Similar to FlatRelatedFields, but combines multiple columns into a single dict to pass to a JSONField on the current model instead. Columns are passed in as `myfield_a`, `myfield_b`, `myfield_c`, which is converted to `myfield = {'a': data, 'b': data, 'c': data}` then saved in the `myfield` JSONField on the model.
- Refactored some magic form code to not pollute the form definition.
- Added tests! 🎉

New arguments to ModelImporter.process:
- allow_updates=True
Allow existing instances to be updated
- allow_inserts=True
Allow the creation of new instances
- limit_to_queryset=None
A queryset of objects that are allowed to be updated

- Fixed another bug with DateTimeParserField where it would prematurely raise an error for an empty field.

- Fixed a bug with stripping `None` in `DateTimeParserField`.