Allow for multiple authors on a single post

This patch allows for multiple authors on a single post

The code isn't pretty and it seems to double the jekyll build time, but it works, and apparently works well.

Code taken from [stackoverflow](http://stackoverflow.com/questions/15189008/how-can-i-have-multiple-authors-for-one-post-in-jekyll). 

To post with multiple authors do 

```
author:
- author1
- author2
```