Tri.query

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* A more helpful error message if you didn't specify `value_to_q_lookup` but you need to.

* Removed buggy use of `setdefaults`. This could cause overriding of nested arguments to not take.

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* Added `Variable.boolean_tristate` for editing boolean fields with null value allowed

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* Made Query a `RefinableObject`

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* Moved to tri.declarative 0.34 and tri.form 5.0

* Note that `gui__class` to specify a constructor/callable is no longer a valid parameter, because of updated tri.form. Use `gui__call_target` or just `gui`.

* You no longer need to specify the `model` parameter to `choice_queryset` or `multi_choice_queryset` if you give it a `QuerySet` as `choices` (as opposed to giving a callable)

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* Move default behaviors out from the shortcuts so they can be used externally

* Removed jquery dependency for toggling simple/advanced filtering

* Removed support for Django 1.7

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* `Variable` class now inherits from `object`, making the implementation more pythonic.
(Attributes still possible to override in constructor call, see `NamespaceAwareObject`)