Ability to add headline image to posts

Allows the user to add headline images to a post. Only accepts image URLs for now. Might look into upload functionality at a later stage.

![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)