Piccolo-api

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Added a ``register`` endpoint, which is great for quickly prototyping a sign up
process (courtesy sinisaos).

Added hooks to the ``session_login`` endpoint, allowing additional logic to be
triggered before and after login.

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Fixing the ``ids`` endpoint of ``PiccoloCRUD`` when a custom primary key column
is used with a name other than ``id``.

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The schema endpoint of ``PiccoloCRUD`` now returns the primary key name. This
means we'll be able to support tables with a custom primary key name in Piccolo
Admin.

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Make sure tables with a custom primary key column work with ``PiccoloCRUD``.

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Fixed a bug with ``PiccoloCRUD``, where a PATCH request returned a string
instead of a JSON object. Thanks to trondhindenes for discovering and fixing
this issue.

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Fixed bug with ``__range_header=false``.

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