Resolve Safari SVG rendering

* Remove old Middleman brand assets
* Move `middleman-logo.svg` outside of `logos/`
* Modify box-shadow value of global nav style

https://trello.com/c/YdBlvKwh

![screen shot 2014-11-11 at 10 27 37 am](https://cloud.githubusercontent.com/assets/816402/4998509/c4dde496-698d-11e4-84ce-fa7bf5aafaa6.png)