don't require directory_watcher unless it's needed

bin/jekyll

@@ -69,6 +69,8 @@ case ARGV.size
 end
 
 if options[:auto]
+  require 'directory_watcher'
+
   puts "Auto-regenerating enabled: #{source} -> #{destination}"
   
   dw = DirectoryWatcher.new(source)

lib/jekyll.rb

@@ -17,7 +17,6 @@ begin
 rescue LoadError
   puts "The rdiscount gem is required for markdown support!"
 end
-require 'directory_watcher'
 
 # internal requires
 require 'jekyll/site'