Improve the registration page:

 * Display developer and commercial differences side by side
 * Display forms only when user has selected a tiers
 * Link to the appropriate page on mashery if the user has a mashery account

![screen shot 2014-07-02 at 17 46 58](https://cloud.githubusercontent.com/assets/615085/3459078/8cac5fcc-0208-11e4-80de-95fa11ecdc1e.png)
![screen shot 2014-07-02 at 17 10 08](https://cloud.githubusercontent.com/assets/615085/3458847/9585fd9a-0205-11e4-942a-1f0a7c29cb6d.png)
![screen shot 2014-07-02 at 17 10 17](https://cloud.githubusercontent.com/assets/615085/3458849/95870096-0205-11e4-8914-6df47e0dbb06.png)
![screen shot 2014-07-02 at 17 10 26](https://cloud.githubusercontent.com/assets/615085/3458848/9586d3b4-0205-11e4-938d-9d2109ec9c0e.png)