S4u.image

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- Add support for hosting images via a separate URL.

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- Add a better index for index for the image scale table.

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- Add support for s4u.upgrade: add an upgrade step to create the image
directories.

- Only use 16 top-level directories; 256 was a bit too much.

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- Move minimal image data to s4u.image.testing to promote reuse by other
packages.

- Register a static view for Pyramid to serve image data.

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- First version.