Adding epub output format, even though it requires a newer version of Sphinx.

Main motivation is people might want to read the documentation for the definitive Amazon AWS interface library, being Boto, on an Amazon Kindle. ePub is easily converted to any other ebook format, such as .mobi for Kindle.
Merge pull request #1759 from geekpete/develop

Adding epub output format, even though it requires a newer version of Sp...