Allow running Enki under a URL path prefix

I run my blog in a subsdirectory of my web server.  That is,
instead of blog.example.com, I use example.com/blog.  Rails 3 no longer
supports that as an out-of-the-box configuration option.

(There is an action_controller.relative_url_root key, but it doesn't
handle asset helper paths or, under some servers, the top-level route
rewriting.)

Tweak routing and URLs in several places:

* config.ru -- addresses most routing
* Set RAILS_RELATIVE_URL_ROOT to make asset helpers work
* Fix Enki-specific URLs in post_path helper and admin view to work
  in a non-root-path deployment

In config/application.rb, comment an example line that sets
RAILS_RELATIVE_URL_ROOT.  The line is commented so that the app still
runs in "/", but a one-line config change or setting that environment
variable would change that.

app/helpers/url_helper.rb

@@ -2,6 +2,7 @@ module UrlHelper
   def post_path(post, options = {})
     suffix = options[:anchor] ? "##{options[:anchor]}" : ""
     path = post.published_at.strftime("/%Y/%m/%d/") + post.slug + suffix
+    path = "#{ENV['RAILS_RELATIVE_URL_ROOT']}#{path}"
     path = URI.join(enki_config[:url], path) if options[:only_path] == false
     path
   end

app/views/layouts/admin.html.erb

@@ -36,7 +36,7 @@
         <li><%= nav_link_to("Health",    admin_health_path,     :accesskey => '6') %></li>
         <li class="separator">&nbsp;</li>
         <li><%= link_to("Logout", admin_session_path, :accesskey => 'l', :method => :delete) %></li>
-        <li><%= link_to("View Site", '/',             :accesskey => 'v', :title => 'View Site') %></li>
+        <li><%= link_to("View Site", root_path,       :accesskey => 'v', :title => 'View Site') %></li>
       </ul>
     </div>
   </div>

config.ru

@@ -1,4 +1,7 @@
 # This file is used by Rack-based servers to start the application.
 
 require ::File.expand_path('../config/environment',  __FILE__)
-run Enki::Application
+
+map ENV['RAILS_RELATIVE_URL_ROOT'] || "/" do
+  run Enki::Application
+end

config/application.rb

@@ -20,6 +20,11 @@
   # Bundler.require(:default, :assets, Rails.env)
 end
 
+# This configures the base path of routes for the main application.
+# For example, set to '/blog' to run at http://example.com/blog
+# It must appear before the Application class body.  Initializers run too late.
+#ENV['RAILS_RELATIVE_URL_ROOT'] = '/blog'
+
 module Enki
   class Application < Rails::Application
     # Settings in config/environments/* take precedence over those specified here.