added tumblr and a bit of documentation to the social button page

Adding a bit of documentation to the social buttons page. Also, have added the tumblr button as an option. It can be hidden w/the `hide: []` property for apps that don't use it.

~~Todo: **We need a tumblr svg**. Is this in existence or in the works?~~

![screenshot 2014-08-25 16 54 03](https://cloud.githubusercontent.com/assets/1597375/4037300/94fa7ee2-2ca2-11e4-82bb-579479f1892d.png)