Added plugin: pelican comment system

The pelican comment system allows you to add static comments to your articles. 
The comments are styleable via the article theme.
It also supports replies to comments.

It is based on the idea of the [static comments](https://github.com/getpelican/pelican-plugins/tree/master/static_comments) plugin.

Example: [blog.scheirle.de](http://blog.scheirle.de/posts/2014/March/29/static-comments-via-email/)