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

test_data/content/pages/untranslated-page.rst

@@ -0,0 +1,5 @@
+Untranslated page
+=================
+:lang: en
+
+This page has no translation.

test_data/content/translated_article-cz.rst

@@ -0,0 +1,8 @@
+Přeložený článek
+================
+:slug: translated-article
+:lang: cz
+:date: 2014-09-15
+
+Jednoduchý článek s překlady.
+Zde je odkaz na `nějaký obrázek <{filename}/images/img.png>`_.

test_data/content/translated_article-de.rst

@@ -0,0 +1,8 @@
+Ein übersetzter Artikel
+=======================
+:slug: translated-article
+:lang: de
+:date: 2014-09-14
+
+Ein einfacher Artikel mit einer Übersetzung.
+Hier ist ein Link zur `einigem Bild <{filename}/images/img.png>`_.

test_data/content/translated_article-en.rst

@@ -0,0 +1,8 @@
+A translated article
+====================
+:slug: translated-article
+:lang: en
+:date: 2014-09-13
+
+A simple article with a translation.
+Here is a link to `some image <{filename}/images/img.png>`_.

test_data/content/untranslated_article-en.rst

@@ -0,0 +1,9 @@
+An untranslated article
+=======================
+:date: 2014-07-14
+:lang: en
+
+An article without a translation.
+Here is a link to an `untranslated page`_
+
+.. _`untranslated page`: {filename}/pages/untranslated-page.rst