WebStyle: canonical and alternate URLs

* Introduces support for canonical URLs, useful in case the same Invenio
  installation is reachable from different domains and over both
  HTTP and HTTPS. (addresses #1251)

* Similarly introduces support for alternate URLs (WRT i18n) to announce
  alternative translation for the current page.

* Introduces proper support for canonical and alternate URLs in the sitemap
  generation script in BibExport.

* Generally improves BibExport generation of sitemaps, by compressing via
  gzip, incremental creation of sitemaps, and hot-swapping.
  (closes #368)

Reviewed-by: Tibor Simko <tibor.simko@cern.ch>