Use checkboxes to represent boolean values

Before:
![Screen Shot 2013-04-17 at 10 47 51 AM](https://f.cloud.github.com/assets/32007/390199/b0654736-a736-11e2-8a93-df13e4d24571.png)

After:
![Screen Shot 2013-04-17 at 10 47 31 AM](https://f.cloud.github.com/assets/32007/390200/b1b2ffe8-a736-11e2-92af-0547cae08e43.png)