Flask-fs2

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- Added backend level configuration ``FS_{BACKEND_NAME}_{KEY}``
- Improved backend documentation
- Use setuptools entry points to register backends.
- Added `NONE` extensions specification
- Added `list_files` to `Storage` to list the current bucket files
- Image optimization preserve file type as much as possible
- Ensure images are not overwritted before rerendering

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- Switch to pytest
- ``ImageField`` optimization/compression.
Resized images are now compressed.
Default image can also be optimized on upload with ``FS_IMAGES_OPTIMIZE = True``
or by specifying `optimize=True` as field parameter.
- ``ImageField`` has now the ability to rerender images with the ``rerender()`` method.

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- Expose Python 3 compatibility

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- Proper github publication
- Initial S3, GridFS and Swift backend implementations
- Python 3 fixes

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- Initial release