Improved checkbox and select accessibility (focuses)

Refactored the checkboxes and select elements for better accessibility via keyboard (focuses) and some other minor Sass fixes (the ampersand...). 

![ghost admin - checkbox and select](https://cloud.githubusercontent.com/assets/1894206/3085912/44f361c8-e51e-11e3-8c5b-da2ad4e2d516.png)