Ad labelling ab test

![ad-label-desktop](https://f.cloud.github.com/assets/1148714/2127981/48122b7e-9275-11e3-8c97-d62d0e21599c.png)
![ad-label-mobile](https://f.cloud.github.com/assets/1148714/2127982/4821b0f8-9275-11e3-8580-4cedda2c7e3f.png)

- AB test to measure the impact on CTR when ads are labelled
- 30% traffic allocation
- Measuring by using difference ad servers (alpha1/alpha2)
- Only run when ads are switched on and viewing an article