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