Add "feedwordpress" and "google-sitemap-generator" WP plugins/ submodules

For OER Impact Hub - includes mods. to Makefile - $(PLUGIN_DIR)

* FeedWordPress 2013.0504 (git code: b4640b6d) 2013-05-04, @radgeek,
  https://github.com/wp-plugins/feedwordpress/releases/tag/2013.0504
  https://github.com/wp-plugins/feedwordpress/commit/b4640b6d7204

* Google XML Sitemaps 3.3.1 (git code: 2f4205cc) 2013-11-26, @arnee,
  https://github.com/wp-plugins/google-sitemap-generator/releases/tag/3.3.1
  https://github.com/wp-plugins/google-sitemap-generator/commit/2f4205cc9625