Added framework to support twitter cards

Established the framework to support Twitter cards.  I only added the twitter:card property which will be set to summary if the opengraph_default_type == article.  This is the major tag that needs to be added to support Twitter cards, as the other properties fail over to the open graph equivalents or are optional.

Relevant Twitter documentation:
https://dev.twitter.com/docs/cards
https://dev.twitter.com/docs/cards/markup-reference
https://dev.twitter.com/docs/cards/validation/validator