Fix exclude to be an empty array

if no exclude is defined in the config file, the exclude should
simply be an empty array.

lib/jekyll/site.rb

@@ -17,7 +17,7 @@ module Jekyll
       self.pygments        = config['pygments']
       self.permalink_style = config['permalink'].to_sym
 
-      self.exclude         = config['exclude']
+      self.exclude         = config['exclude'] || []
 
       self.reset
       self.setup