Allow 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.

Also nothing gets broken, which is even better.

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
```