Add a twitter icon to author links ONLY when theme_preview is active

This changeset continues existing functionality for the current themes:

![screen shot 2014-09-30 at 2 51 15 pm](https://cloud.githubusercontent.com/assets/430385/4464328/d182e1f0-48d2-11e4-85ed-33b321f96929.png)

But adds the new elements when the appropriate plugins and functions exist:

![screen shot 2014-09-30 at 2 54 40 pm](https://cloud.githubusercontent.com/assets/430385/4464383/400a1bf2-48d3-11e4-9e43-8d1b3fb5fcdf.png)


See: https://github.com/GigaOM/gigaom/issues/5440