Add I18n interface for use in Mustache templates (#1679)

* Add `i18n` dependency

We will use `i18n` to provide localization for Gollum's frontend. I
chose this because it's a well-supported, pretty normal Ruby library.

* Configure I18n

- Locale files will be kept in `lib/gollum/locales/[lang].yml`
- The available locales, to start, will be English (`en`).

* Add I18n interface for mustache templates

This commit adds an interface that allows mustache templates to get I18n
translation strings, transform any arguments that may be present in
them, and then render them on the frontend.

This is our first real step to getting internationalizing the Gollum
frontend.

lib/gollum.rb

@@ -5,6 +5,7 @@ require 'digest/sha1'
 require 'ostruct'
 
 # external
+require 'i18n'
 require 'github/markup'
 require 'rhino' if RUBY_PLATFORM == 'java'
 
@@ -14,10 +15,13 @@ require File.expand_path('../gollum/uri_encode_component', __FILE__)
 module Gollum
   VERSION = '5.2.3'
 
+  ::I18n.available_locales = [:en]
+  ::I18n.load_path = Dir[File.expand_path("lib/gollum/locales") + "/*.yml"]
+
   def self.assets_path
     ::File.expand_path('gollum/public', ::File.dirname(__FILE__))
   end
-  
+
   class TemplateFilter
     @@filters = {}
 
@@ -32,5 +36,4 @@ module Gollum
       data
     end
   end
-
 end