SitemapFacter: Fixing XMLSyntaxError

xml.etree in lxml.etree._raiseParseError (src/lxml/lxml.etree.c:91772)
XMLSyntaxError: PCDATA invalid Char value 31, line 26, column 23453
User: Refactoring authenticate
Include domain_name in the index (fix #120)
Support authentication & authorization over OAuth (fix #121)

Side effects:
 * Update docs
 * Update config
 * Allow specifying batch size in command line while indexing
 * pyelasticsearch's `_bulk` POSTs to the ES API root and this breaks
   authentication/authorization through Backstage. Consequently,
   the mapping and the indexing process had to be changed
Draft a new release, 0.13.4