Update AutocompleteModel and AutocompleteGeneric:

- Enable split_words='or' to use "OR" conditions on splitted words.
- Set order_by=None by default in AutocompleteModel.
- Added missing docstrings.
- Made it easier to spot failing tests.
- Covered all combinations with tests.
- Set split_words=False to default again.
Version bump 1.4.0