Adds label's layer -masksToBounds to fix round corner issues

Before this change, the badge label was showing as a rect with no rounded corners:
![image](https://f.cloud.github.com/assets/590579/2403892/85e38372-aa34-11e3-8086-0806584a33f0.png)

Then, when adding:
```
self.badge.layer.masksToBounds = YES;
```

The result was looking better:
![image](https://f.cloud.github.com/assets/590579/2403901/ab0390fc-aa34-11e3-9d55-fd0aea6cba06.png)