Tri-query

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* Report invalid operators for choice queryset as errors instead of crashing

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* Check for dates that are out of range

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* Cache form on Query

* Render global form errors

* Query.form() no longer takes a request parameter, since it was redundant anyway. This is a potential breaking change.

* Correctly dispatch attr to underlying tri.form

* Support for ajax backend. New parameter to Query: endpoint_dispatch_prefix.

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* Minor bugfix for variable-from-model handling of auto fields

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* Fix over-eager assert not allowing variable-less Query objects.

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* Enable mixing variable definitions in both declared variables and class meta.

* Added `after` attribute on `Variable` to enable custom variable ordering (See `tri.declarative.sort_after()`)

* Added Variable.from_model, Query.from_model