20140223 modal byline

The modal for photos always tries to display the twitter handle, but with Instagram or RSS it displays a broken "@" link. Let's use an existence test before creating that link.

Before:
![image](https://f.cloud.github.com/assets/1263746/2240514/ab0214a2-9c97-11e3-80d9-8c1805bbb34c.png)

After:
![image](https://f.cloud.github.com/assets/1263746/2240522/f5bd6096-9c97-11e3-8e67-a76dc927befe.png)