Fix checkboxes style

Reduced size of checkboxes and aligning it to the left.

Also remove extra shadow (the gray line above the "Before" screenshot).

Before:
![screenshot-by-nimbus 13](https://cloud.githubusercontent.com/assets/688373/4699840/64ffd0e6-5846-11e4-98b7-1f6ccb0dfbd2.png)

Now:
![screenshot-by-nimbus 14](https://cloud.githubusercontent.com/assets/688373/4699844/685ac2dc-5846-11e4-87d2-a18d1588a168.png)