Fixed parsing of epubcheck reports and made html prettier

The format of the messages in the XML output of epubcheck has changed since version 3. This PR fixes the parsing so that it works in the new versions of epubcheck. Tested against epubcheck 4.0.0-alpha12-SNAPSHOT (see IDPF/epubcheck#471).

The HTML reports are also made slightly prettier and more aligned with the reports produced by http://validator.idpf.org/ (i.e. table format).

Depends on daisy/pipeline-scripts-utils#59