Allowing to use the same sideload key in multiple transformers

Sometimes there is a relationship in which Model 1 belongs to Model 2 which has Model 1. If one try to write transformer for them the key from the last transformer is going to replace first. I know, hard to understand. Maybe an example?

![diagram](https://cloud.githubusercontent.com/assets/5459938/5140733/0ac31700-716e-11e4-9fcc-ddbe69d3a9c2.png)

And situation in which one is going to display a list of urls with urls that are redirecting to them. 
This pull request fix the problem.