Add an author_bio variable to allow user's to have a bio

svbtle now has a short bio for the author written under their name. it would be cool if people could add their own bio (I used the AUTHOR_BIO variable). 
![bio](https://f.cloud.github.com/assets/391240/322946/fb4700ec-9a88-11e2-9c06-16ed9f670d2b.png)