UITableViewCell background color in grouped table

This is a small change to the way a background color is set in a ```UITableViewCell```.

Previously: a new view is created with the required background color. This view is rectangular, so ruins the rounded effect of a ```UITableViewCell``` inside a grouped table.

This change just sets the ```backgroundColor``` property on the cell itself. 

Before and after:
![image](https://f.cloud.github.com/assets/355086/957126/52be50a6-044b-11e3-8be9-ed9fb3a415e4.png)
![image](https://f.cloud.github.com/assets/355086/957130/693c5de6-044b-11e3-8791-008353e3e871.png)