Feature/logo url

User needs to know what size application image to provide #360
![screen shot 2014-09-25 at 9 28 31 pm](https://cloud.githubusercontent.com/assets/42814/4414680/e0cd85ac-451c-11e4-9e0f-686a140fcfbc.png)

Best to use `em` instead of `px` since that's what we've been using.