Only load Asset Sync when S3 is configured

I consider this a hack, it needs review so I won't push it straight
to the main repo. I tried putting the same code in an initializer but
that appears to be too late to properly initialize the asset_sync
engine.

See diaspora/diaspora#3102

Gemfile

@@ -119,7 +119,9 @@ group :assets do
   
   gem 'handlebars_assets'
   gem 'uglifier'
-  gem "asset_sync"
+  
+  # asset_sync is required as needed by application.rb
+  gem "asset_sync", :require => nil
 end
 
 gem 'jquery-rails'

config/application.rb

@@ -14,9 +14,12 @@ end
 
 require 'rails/all'
 
+# Sanitize groups to make matching :assets easier
+RAILS_GROUPS = Rails.groups(:assets => %w(development test)).map { |group| group.to_sym }
+
 if defined?(Bundler)
   # If you precompile assets before deploying to production, use this line
-  Bundler.require(*Rails.groups(:assets => %w(development test)))
+  Bundler.require(*RAILS_GROUPS)
   # If you want your assets lazily compiled in production, use this line
   # Bundler.require(:default, :assets, Rails.env)
 end
@@ -90,3 +93,8 @@ module Diaspora
 
   end
 end
+
+# Only load asset_sync if S3 is configured
+if RAILS_GROUPS.include?(:assets) && ENV['AWS_ACCESS_KEY_ID']
+  require 'asset_sync'
+end