Use post.excerpt as summary and add DISGUS support

For better SEO and more flexibility
Changed to use [Post excerpts](http://jekyllrb.com/docs/posts/#post-excerpts) in the **description meta-tag**
Index.html also use post exceprt as post summary
So users can customize excerpt on front matter and it will be reflected on index and SEO


Also add a new config parameter: **disgus**
site with this parameter will try to load and display disgus at the end of post