Add twitter button to bottom of blog posts

Needed for https://github.com/CocoaPods/blog.cocoapods.org/issues/47

![screen shot 2014-10-10 at 2 55 49 pm](https://cloud.githubusercontent.com/assets/1946610/4599599/58f56f8e-50c8-11e4-971d-8762c5d91fed.png)