add duoshuo comment provider for chinese users

the reason is disqus and many other comment provider are too slow in china. 
duoshuo is a new comment provider in china(but not only open to china, it is becomming more and more popular in the world http://duoshuo.com) similar to disqus.  it can be installed on many platform,such as wordpress, mediawiki, dedecms, phpcms, discuz!x, joomla and so on

many jekyll users are using duoshuo too.
so, i think duoshuo should be added to jekyll-bootstrap for more users ;)