Tri-form

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* `Field.from_model` produced incorrect parser for foreign keys.

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* Correctly check unique constraints when editing object

* Bug where `form.instance` was set too late in `Form` constructor, resulting in `AttributeError` in certain lambdas

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* Refactored to use new tri.declarative: using `RefinableObject`, `Shortcut`, and the new `call_target` parameter of `Namespace`

* Changed dispatch separator from `__` to `/`

* Changed `label` on `Field` to `display_name`. This is in line with tri.query and tri.table.

* `Field.choice` incorrectly handles post_validation (fixes github issue 9)

* `Form` now doesn't try to read initial data and form data from Fields that are show=False

* `create_or_edit_object` doesn't take a parameter `render_context` anymore, use `render__context`

* 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)

* `Field.choice_queryset` and `Field.multi_choice_queryset` now reevaluates the QuerySet on each usage (fixes tri.table github issue 16)

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* Bugfix: `on_save` callback for `create_or_edit_object` was missing `form` parameter

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* Fixed bug where namespaced form was not rendered when the other namespaced form had an error

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* Added attribute 'name' to Form in order to namespace POSTs in cases where there are more than one form