Dataflake.docbuilder

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- Bug: If the ``z3csphinx-output-directory`` was set, all its
contained packages ended up on the index document. Now this
only happens if no SVN source URLs are otherwise provided.
If they are, only packages from those source URLs are
considered for linking on the index document.

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- Feature: If you generate some documentation via
`z3c.recipe.sphinxdoc` and want to stitch links to it
into the generated index file, you can use the new
``z3csphinx-output-directory`` parameter to point the script
to the generated package documentation root folder.

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- Bug: Don't clean up intermediate files, otherwise it is not
possible to re-use a template folder for creating several
separate pages into an output folder.

- Bug: Clean up group header creation to avoid header level
mixups.

- Bug: When creating a missing required index.rst, use a
template file if it exists.

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- Feature: Added a script and buildout option ``index-name`` to
specify the file name (without extension) for the index page.
With this option you can safely build the index page into an
existing `Sphinx` documentation folder without overwriting
or changing the existing ``index.rst`` file and its HTML
equivalent. The default continues to be ``index.rst``, though.

- Feature removed: It is no longer possible to create a simple HTML
index page without using `Sphinx` and a minimal `Sphinx`
configuration.

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- Feature: Add new script option ``-g``/``--grouping`` and zc.buildout
option ``grouping`` to group packages.

- Miscellaneous: Renamed the zc.buildout option `source` to `sources`
since it contains one or more elements.

- Miscellaneous: Removed the version pinning on the Sphinx dependency
since our other dependency (repoze.sphinx.autointerface) is now
compatible with Sphinx 1.0.

- Bug: If pkg_resources.find_distributions cannot find valid
Egg distributions we still force the tag folder itself into the
pkg_resources.working_set as a fallback.

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- Feature: The user can now provide a Sphinx configuration folder
path that will be used to generate additional content for the
documentation root folder.

- Factoring: Moved the DocsBuilder class into its own module.

- Factoring: Save run state on the documentation builder class
instead of handing it around

- Cosmetic: Use a flat hierarchy when creating the HTML output links
instead of a folder per package. Only a single index page needs to
be created that way.