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/taxonomy_test.go

@@ -20,18 +20,6 @@ import (
 	"github.com/spf13/viper"
 )
 
-func TestSitePossibleTaxonomies(t *testing.T) {
-	site := new(Site)
-	page, _ := NewPageFrom(strings.NewReader(pageYamlWithTaxonomiesA), "path/to/page")
-	site.Pages = append(site.Pages, page)
-	taxonomies := site.possibleTaxonomies()
-	if !compareStringSlice(taxonomies, []string{"tags", "categories"}) {
-		if !compareStringSlice(taxonomies, []string{"categories", "tags"}) {
-			t.Fatalf("possible taxonomies do not match [tags categories].  Got: %s", taxonomies)
-		}
-	}
-}
-
 func TestByCountOrderOfTaxonomies(t *testing.T) {
 	viper.Reset()
 	defer viper.Reset()