Added microdata, html5 tags

This is what I added:

- fixed typos in templates/imprint.php
- added theme option under tab Social Media for the federated social network Friendica in inc/constants.php
- added function piratenkleider_html_tag_schema to inc/functions.php
- added following microdata object types in html tag:
  - WebPage
  - Article
  - AboutPage
  - ContactPage
  - Person
- added theme option under tab Misc to assign object types AboutPage and ContactPage to pages determined by the admin
- added type Video for embedded videos, property: embedUrl
- added microdata properties for object type Article: name, articleBody, articleSection, creator, keywords, url
- added microdata properties for object type Person: mainContentOfPage, name, description, email, url
- added microdata properties for object type WebPage/AboutPage/ContactPage: mainContentOfPage, name, image, text
- added microdata for comments meta: comment, interactionCount: content=Usercomments, discussionUrl
- added microdata for comments: commentText, name, commentTime, url, image
- added HTML5 article and header-tags to comments
- other microdata added: breadcrumb, datePublished, image, SiteNavigationElement
- added role="navigation" to main navigation in header.php