Twitter Card tags now find in-content images & updated to new twitter spec

* :image >> :image:src
* in-content images, w/ duplicate prevention
* meta tag format normalized
* required site domain tag added
Prevents empty img:src attribute in Twitter Card tags
(possible, micro-perf boost by preventing an in_array() call)
Merge pull request #29 from mbijon/master

Twitter Card tags updated: Adds in-content images & new tag specs