adds conditional for recovering dir paths fixes #22

The process of creating a relative path from an absolute one seems to
strip the trailing slash, identifying a path as a directory not a file.

When this is converted into a Regex for adding to the ignore list, it
also ignores files that start with the same name as ignored directories.

This fix simply checks whether there was originally a trailing slash,
then adds it again before it is converted into a regular expression.

lib/jekyll/watcher.rb

@@ -78,13 +78,21 @@ def config_files(options)
     end
 
     def to_exclude(options)
+      # prepending a slash to the destination directory will ensure that
+      # other files starting with _site aren't also ignored by Listen
+      destination = "#{options['destination']}/"
+
       [
         config_files(options),
-        options["destination"],
-        custom_excludes(options),
+        destination,
+        custom_excludes(options)
       ].flatten
     end
 
+    def has_trailing_slash(path)
+      path[-1] == '/'
+    end
+
     # Paths to ignore for the watch option
     #
     # options - A Hash of options passed to the command
@@ -95,14 +103,19 @@ def listen_ignore_paths(options)
       paths        = to_exclude(options)
 
       paths.map do |p|
+        is_dir = has_trailing_slash(p)
         absolute_path = Pathname.new(p).expand_path
         next unless absolute_path.exist?
-        begin
-          relative_path = absolute_path.relative_path_from(source).to_s
-          unless relative_path.start_with?("../")
-            path_to_ignore = Regexp.new(Regexp.escape(relative_path))
-            Jekyll.logger.debug "Watcher:", "Ignoring #{path_to_ignore}"
-            path_to_ignore
+          begin
+            relative_path = absolute_path.relative_path_from(source).to_s
+            recovered_path = is_dir ? "#{relative_path}/" : relative_path
+            unless recovered_path.start_with?('../')
+              path_to_ignore = Regexp.new(Regexp.escape(recovered_path))
+              path_to_ignore
+              Jekyll.logger.debug "Watcher:", "Ignoring #{path_to_ignore}"
+            end
+          rescue ArgumentError
+            # Could not find a relative path
           end
         rescue ArgumentError
           # Could not find a relative path