fixed markup and broken links

Checking the site with W3C Validator i found a few markup errors (unclosed tags in template files etc.). Also corrected a few links to sample project source.

Edit: I had to revert the Page.full_path method in the articles_tag plugin. My local jekyll (0.11) rendered page.url to the filename only and omited the path completely. Just checked the project Gemfile and updated to the current jekyll (1.2).