Sbo-sphinx

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* Cleaner EPUB output by default, including a default cover
* Added support for CommonMark-flavored Markdown source files by default
* Better error message from the jsdoc extension when ant is not installed
* Updated sbo-sphinx/conf.py to reflect what's generated by Sphinx 1.3
* setup.py no longer installs exact versions of dependencies in Read the Docs
builds (it caused problems when installed as a dependency of another
package). Specify a requirements file in the Read the Docs configuration
instead if exact versions are desired.

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* Upgraded to a version of sphinx_rtd_theme in which html_logo works correctly
* Assorted packaging cleanup

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* Removed validate_readme.py in favor of "python setup.py check --restructuredtext --strict"
via the "readme" package
* Switched Python & Django intersphinx links to HTTPS
* Python 3 support
* Assorted packaging cleanup

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Fixed compatibility with pip 6.x

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Minor doc and packaging updates

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* Added support for custom favicon and logo images
* Added PyPI description validator
* Packaging cleanup