Fix modal edge spacing

No issue

The transparent border surrounding modals was being cut off.
This fix uses padding instead of adjusting the left & right valued to create space.

Before: 
![pasted_image_at_2014_12_23_13_49_360](https://cloud.githubusercontent.com/assets/390392/5537584/37f2fad2-8a9c-11e4-93d3-d3d857f4233d.png)

After:
![screen shot 2014-12-23 at 12 06 27](https://cloud.githubusercontent.com/assets/390392/5537587/3e151e4a-8a9c-11e4-8444-2d5cfe62e536.png)