Add Documentation for Icons to Styleguide

Also add new Vellum Icon for Signature
![screen shot 2014-11-05 at 3 11 46 pm](https://cloud.githubusercontent.com/assets/716573/4925079/3c0e17ee-6528-11e4-8725-a6b516c3b92b.png)
![screen shot 2014-11-05 at 3 11 54 pm](https://cloud.githubusercontent.com/assets/716573/4925082/4094491e-6528-11e4-825f-33790e4c8612.png)