dtbook-to-epub3: valid epub output. html splitter refactoring and tests.

* content files stored in /EPUB/ instead of /EPUB/Content/. There won't be any naming conflicts with nav.xhtml, ncx.xml or css/ anyway.
* fixed image files not being copied
* adding id attributes to all elements except span, p, div, tr, th and td (more exceptions can be added if desired)
* HTML splitter split into two XSLT steps instead of one (first step sets base URIs for sections so that it's easier to resolve links in the second step)
* output from splitting is now given on the primary output wrapped in a wrapper element instead of using xsl:result-document. This makes it possible to test using XSpec
* added xspec tests for the two html splitting xslt steps
* made sure that the output epub is valid when using the DAISY test book Valentin Hauy