Twitter Card support.
Wordpress import: do not write 'None' as a description if it isn't present.
Support Open Graph aswell.
Updated documentation.
Merging https://github.com/ralsina/nikola master
Merge branch 'master' of https://github.com/ralsina/nikola into twitter_cards
Small improvements based on Robertos review.
Merge pull request #317 from okin/twitter_cards

Twitter Card support