Epub: Added option to include HTML Table of Contents in ebook

Normally, an epub ebook does not include an HTML Table of Contents because the reading device or application makes better use of the navigational TOC (defined in the toc.ncx file). It may be cases, though, when it may be convenient or even necessary to include one, as when the epub file will be later converted to mobi format (for Amazon Kindle) using Amazon's official converter **kindlegen**. Amazon clearly states in its *Kindle Publisihg Guidelines* (http://kindlegen.s3.amazonaws.com/AmazonKindlePublishingGuidelines.pdf) that all books *must* include an HTML TOC in addition of the navigational TOC.

This PR adds an additional option to generate and include an HTML TOC (default to false). Both HTML and CSS code are taken from examples in Kindle Publishing Guidelines.  

Documentation files are also modified.