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)
Also adds the Dashboard icons:
![screen shot 2014-11-05 at 5 15 59 pm](https://cloud.githubusercontent.com/assets/716573/4927200/6b1e605a-6539-11e4-87b1-7cc3e1570aba.png)
And a couple vellum icons to address http://manage.dimagi.com/default.asp?85272 (cos why not)
![screen shot 2014-11-05 at 5 16 13 pm](https://cloud.githubusercontent.com/assets/716573/4927211/80f7e43c-6539-11e4-97f3-55b571839375.png)
![screen shot 2014-11-05 at 5 16 09 pm](https://cloud.githubusercontent.com/assets/716573/4927222/923a293a-6539-11e4-9869-9c81232ef171.png)

![screen shot 2014-11-05 at 3 11 54 pm](https://cloud.githubusercontent.com/assets/716573/4925082/4094491e-6528-11e4-825f-33790e4c8612.png)