Relief

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- Split :class:`Element` up into :class:`BaseElement`, :class:`NativeMixin`,
:class:`ValidatedByMixin` and :class:`DefaultMixin`.
- Add :class:`Maybe`.
- Add :class:`relief.validation.MatchesRegex`.
- Add :class:`relief.validation.IsURL`.

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- Pass :data:`Unspecified` to :meth:`Element.serialize` and
:meth:`Element.serialize`, when setting values.
- Don't ignore defaults of members in :class:`Form`.

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- Turn :meth:`Element.unserialize` into an instance method.
- Rename :meth:`Element.set` to `Element.set_from_raw`.
- Add :meth:`Element.serialize` and :meth:`Element.set_from_native`.
- Make :data:`Unspecified` and :data:`NotUnserializable` false-ish.
- Make ``unicode(Unspecified)`` and ``unicode(NotUnserializable)`` return an
empty string.
- Make ``bytes(Unspecified)`` and ``bytes(NotUnserializable)`` return an
empty string.

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- Values in :class:`~relief.Form` structures can now be validated by
subclassing :class:`~relief.Form` and adding `validate_{key}` methods.

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- Removed mention of being able to mutate :class:`relief.Dict` and
:class:`relief.Form`.

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Initial release.