Adds support for related posts plugin

Adds a new template/include/ file to be used with the related posts
plugin (http://github.com/getpelican/pelican-plugins). It also adds two
new variables: SHOW_RELATED_POSTS and RELATED_POSTS_TEXT, witch will
enable related posts to be shown at the end of the article and customize
the text right above the related posts.
Removes the SHOW_RELATED_POSTS flag
Merge pull request #35 from magnunleno/related-posts

Adds support for related posts plugin