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/localized_theme/babel.cfg

@@ -0,0 +1,2 @@
+[jinja2: templates/**.html]
+

test_data/localized_theme/messages.pot

@@ -0,0 +1,23 @@
+# Translations template for PROJECT.
+# Copyright (C) 2014 ORGANIZATION
+# This file is distributed under the same license as the PROJECT project.
+# FIRST AUTHOR <EMAIL@ADDRESS>, 2014.
+#
+#, fuzzy
+msgid ""
+msgstr ""
+"Project-Id-Version: PROJECT VERSION\n"
+"Report-Msgid-Bugs-To: EMAIL@ADDRESS\n"
+"POT-Creation-Date: 2014-07-13 12:25+0200\n"
+"PO-Revision-Date: YEAR-MO-DA HO:MI+ZONE\n"
+"Last-Translator: FULL NAME <EMAIL@ADDRESS>\n"
+"Language-Team: LANGUAGE <LL@li.org>\n"
+"MIME-Version: 1.0\n"
+"Content-Type: text/plain; charset=utf-8\n"
+"Content-Transfer-Encoding: 8bit\n"
+"Generated-By: Babel 1.3\n"
+
+#: templates/base.html:3
+msgid "Welcome to our"
+msgstr ""
+

test_data/localized_theme/templates/base.html

@@ -0,0 +1,7 @@
+{% extends "!simple/base.html" %}
+
+{% block title %}{% trans %}Welcome to our{% endtrans %} {{ SITENAME }}{% endblock %}
+{% block head %}
+{{ super() }}
+<link rel="stylesheet" href="{{ SITEURL }}/{{ THEME_STATIC_DIR }}/style.css" />
+{% endblock %}

test_data/localized_theme/translations/de/LC_MESSAGES/messages.po

@@ -0,0 +1,23 @@
+# German translations for PROJECT.
+# Copyright (C) 2014 ORGANIZATION
+# This file is distributed under the same license as the PROJECT project.
+# FIRST AUTHOR <EMAIL@ADDRESS>, 2014.
+#
+msgid ""
+msgstr ""
+"Project-Id-Version: PROJECT VERSION\n"
+"Report-Msgid-Bugs-To: EMAIL@ADDRESS\n"
+"POT-Creation-Date: 2014-07-13 12:25+0200\n"
+"PO-Revision-Date: 2014-07-13 12:26+0200\n"
+"Last-Translator: FULL NAME <EMAIL@ADDRESS>\n"
+"Language-Team: de <LL@li.org>\n"
+"Plural-Forms: nplurals=2; plural=(n != 1)\n"
+"MIME-Version: 1.0\n"
+"Content-Type: text/plain; charset=utf-8\n"
+"Content-Transfer-Encoding: 8bit\n"
+"Generated-By: Babel 1.3\n"
+
+#: templates/base.html:3
+msgid "Welcome to our"
+msgstr "Willkommen Sie zur unserer"
+