Custom rendering for checkbox inputs on OS X Yosemite

This change removes the Mac styling for `input type="checkbox"` controls and replaces it with CSS that draws a facsimile. It works around a bug on Yosemite wherein checkbox animations are drawn in the wrong place (and drawn multiple times). 

Before:
![image](https://cloud.githubusercontent.com/assets/470418/4818016/ded0895c-5eee-11e4-8c97-cf070e541736.png)

After:
![image](https://cloud.githubusercontent.com/assets/470418/4818020/ecc6cb48-5eee-11e4-95c4-47b47fcd15bb.png)