Optimize call to canOpenURL: to improve performance

I'm using OHAtributedLabel a lot in the tableviewcells, so recently I've encountered this performance issue during ```commonInit```:
![123](https://f.cloud.github.com/assets/829783/1244788/870b17b8-2a8e-11e3-8a4e-bff68c98d850.png)

I've fixed it by asking the app if it can handle numbers only once.