html-to-epub3: always set language of content docs

The language is set as `@lang` and `@xml:lang` on the `html` root, from existing
attributes or from the system language is no language is declared.
Improve gitignore filtering of XSpec results
Allow fileset resource manipilation the html-to-epub3

- the HTML resources are passed to `pxi:html-to-epub3-content`
- `pxi:html-to-epub3-content` now outputs two filesets: one for content
  docs and one for resources
- the next steps use the "filtered" resource fileset created by
  `pxi:html-to-epub3-content` instead of the source HTML resources
  fileset
Properly convert image descriptions to EPUB3

- resource manipulation is now done in `html-to-epub3.content.xpl`
- resources URIs are sanitized (safe URIs, remove non-existing)
  - see `html-clean-resources.xsl`
- if image descriptions are found, try to convert DIAGRAM to HTML
  - see `convert-diagram-descriptions.xpl`
  - see `fileset-convert-diagram.xsl`
  - see `diagram-to-html.xsl`
- @longdesc and @aria-describedat are converted and/or doubled with
  @aria-describedby pointing to hidden asides with iframes.
- more tests
Merge pull request #41 from daisy/feature/html-image-desc

Properly convert @longdesc and @aria-describedat to EPUB 3