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/embedded_shortcodes_test.go

@@ -56,8 +56,8 @@ func doTestShortcodeCrossrefs(t *testing.T, relative bool) {
 	templ := tpl.New()
 	p, _ := pageFromString(simplePageWithURL, path)
 	p.Node.Site = &SiteInfo{
-		Pages:   &(Pages{p}),
-		BaseURL: template.URL(helpers.SanitizeURLKeepTrailingSlash(baseURL)),
+		AllPages: &(Pages{p}),
+		BaseURL:  template.URL(helpers.SanitizeURLKeepTrailingSlash(baseURL)),
 	}
 
 	output, err := HandleShortcodes(in, p, templ)