Add multilingual support in Hugo

Implements:
* support to render:
  * content/post/whatever.en.md to /en/2015/12/22/whatever/index.html
  * content/post/whatever.fr.md to /fr/2015/12/22/whatever/index.html
* gets enabled when `Multilingual:` is specified in config.
* support having language switchers in templates, that know
  where the translated page is (with .Page.Translations)
  (when you're on /en/about/, you can have a "Francais" link pointing to
   /fr/a-propos/)
  * all translations are in the `.Page.Translations` map, including the current one.
* easily tweak themes to support Multilingual mode
* renders in a single swift, no need for two config files.

Adds a couple of variables useful for multilingual sites

Adds documentation (content/multilingual.md)

Added language prefixing for all URL generation/permalinking see in the
code base.

Implements i18n. Leverages the great github.com/nicksnyder/go-i18n lib.. thanks Nick.
* Adds "i18n" and "T" template functions..

hugolib/translations.go

@@ -0,0 +1,59 @@
+// Copyright 2016 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package hugolib
+
+// Translations represent the other translations for a given page. The
+// string here is the language code, as affected by the `post.LANG.md`
+// filename.
+type Translations map[string]*Page
+
+func pagesToTranslationsMap(pages []*Page) map[string]Translations {
+	out := make(map[string]Translations)
+
+	for _, page := range pages {
+		base := page.TranslationBaseName()
+
+		pageTranslation, present := out[base]
+		if !present {
+			pageTranslation = make(Translations)
+		}
+
+		pageLang := page.Lang()
+		if pageLang == "" {
+			continue
+		}
+
+		pageTranslation[pageLang] = page
+		out[base] = pageTranslation
+	}
+
+	return out
+}
+
+func assignTranslationsToPages(allTranslations map[string]Translations, pages []*Page) {
+	for _, page := range pages {
+		base := page.TranslationBaseName()
+		trans, exist := allTranslations[base]
+		if !exist {
+			continue
+		}
+
+		for lang, translatedPage := range trans {
+			if translatedPage == page {
+				continue
+			}
+			page.Translations[lang] = translatedPage
+		}
+	}
+}