Handle empty criteo value

DFP was throwing error, due to us adding an empty targeting param when ctriteo was empty.

Also, tidy up crtieo js by using `url.js`'s functions

![screen shot 2014-07-30 at 16 06 35](https://cloud.githubusercontent.com/assets/74132/3750991/21298320-17fb-11e4-96d6-55a88f8c02e0.png)