Print warning when paginate is set to true.

Related to jekyll#1105.

Sample output:
~/code/jekyll/tsite$ ../bin/jekyll build --trace
Configuration file: /Users/parker/code/jekyll/tsite/_config.yml
    Config Warning: The `paginate` key must be a positive integer or nil. It's currently set to 'true'.
            Source: /Users/parker/code/jekyll/tsite
       Destination: /Users/parker/code/jekyll/tsite/_site
      Generating... done.

lib/jekyll/configuration.rb

@@ -150,7 +150,7 @@ def read_config_files(files)
         $stderr.puts "#{err}"
       end
 
-      configuration.backwards_compatibilize
+      configuration.fix_common_issues.backwards_compatibilize
     end
 
     # Public: Split a CSV string into an array containing its values
@@ -205,5 +205,17 @@ def backwards_compatibilize
       config
     end
 
+    def fix_common_issues
+      config = clone
+
+      if config.has_key?('paginate') && (!config['paginate'].is_a?(Integer) || config['paginate'] < 0)
+        Jekyll.logger.warn "Config Warning:", "The `paginate` key must be a" +
+          " positive integer or nil. It's currently set to '#{config['paginate'].inspect}'."
+        config['paginate'] = nil
+      end
+
+      config
+    end
+
   end
 end