Update the i18n_subsites plugin, addresses many issues

Major highlights
................
- fixed and improved cross-linking (fixes #333) with URLs
  containing e.g. localized month names
  (thanks to issue getpelican/pelican#1198)
- support for custom ``SITEURL`` and ``OUTPUT_PATH`` hierarchy
  (fixes #182)
- sharing of static files (including those of the theme) among
  subsites (fixes #180)

Technical highlights
....................
- added a test suite (works with pelican 3.4)
- translations are installed into Jinja2 environments of all
  generators
- old locale is restored after generation, fixes autoreload

The documentation has been updated and improved (mostly in terms of
formatting).

Known issues
............
- due to the redesign required for correct cross-linking, older
  versions of Pelican (<3.4) are not supported, because they lack
  certain signals
- the ``HIDE_UNTRANSLATED_CONTENT`` setting has been deprecated in
  favor of the ``I18N_UNTRANSLATED_{ARTICLES,PAGES}`` settings which
  offer more control in order to fix #211.
- the test suite works only with pelican 3.4, later versions add a
  timezone field to the date
The multi_part plugin has been set as deprecated
Added series plugin
Merge pull request #346 from lgiordani/series_plugin

New series plugin. Multi_part set as deprecated