Make table headers opaque

@mtedeschi can weigh in on the selected opaque color

Before:
![ios simulator screen shot may 23 2014 8 45 59 am](https://cloud.githubusercontent.com/assets/1818302/3066663/a4ffc3e8-e278-11e3-951d-b438ebc87d59.png)
After:
![ios simulator screen shot may 23 2014 8 44 47 am](https://cloud.githubusercontent.com/assets/1818302/3066665/a71167b8-e278-11e3-9a0b-4ed9468f5621.png)