Webresource

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- Raise explicit ``ResourceError`` instead of generic ``ValueError``.

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- Make ``Resource.directory`` a R/W property.

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- Change ``path`` cascading behavior. Path set on ``ResourceGroup`` always takes
precedence over its members paths.

- ``include`` property of ``Resource`` and ``ResourceGroup`` can be set from
outside.

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- Add remaining missing rst files to release.

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- Add missing ``docs/source/overview.rst`` to release.

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- Initial release.