Alchy

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- Fix ``utils.iterflatten()`` by calling ``iterflatten()`` instead of ``flatten`` in recursive loop.

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- Add ``ModelBase.primary_attrs`` class property that returns a list of class attributes that are primary keys.
- Use ``ModelBase.primary_attrs`` in ``QueryModel.search()`` so that it handles cases where primary keys have column names that are different than the class attribute name.

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- Modify internals of ``QueryModel.search()`` to better handle searching on a query object that already has joins and filters applied.

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- Add ``search.icontains`` and ``search.noticontains`` for case insensitive contains filter.
- Remove strict update support from ``Model.update()``. Require this to be implemented in user-land. (**breaking change**)

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- Merge originating query where clause in ``Query.search`` so that pagination works properly.
- Add ``session_class`` argument to ``Manager`` which can override the default session class used.

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- In ``ModelMeta`` when checking whether to do tablename autogeneration, tranverse all base classes when trying to determine if a primary key is defined.
- In ``ModelMeta`` set ``bind_key`` in ``__init__`` method instead of ``__new__``. This also fixes an issue where ``__table_args__`` was incorrectly assumed to always be a ``dict``.