Ability to add headline image to posts

I needed to be able to add headline images to a post, so I extended Hadron a bit. Only accepts image URLs for now. Might look into upload functionality at a later stage. I thought it might be useful for others.

![image](https://cloud.githubusercontent.com/assets/4135045/2748951/2eb5bdbe-c7e3-11e3-8655-983e2dbdb35f.png)

![image](https://cloud.githubusercontent.com/assets/4135045/2748956/3e8acdba-c7e3-11e3-97f9-893e9c71941b.png)

Related [hadron-theme-nodus PR](https://github.com/hadronjs/hadron-theme-nodus/pull/1) to ensure image doesn't exceed article width.