Enable passing a url as callback url

Currently there is only one callback, the one on the config.
My change is enabling the user to set mulitple callbacks, as the callback is a parameter.
This will help for instance when I have a model ```File``` where I convert a doc to an image,
so the callback on my controller would look like:
```/files/:id/convert```
and I will pass it to cloudconvert as a callback (/files/12/convert).

Also, did some cleanups and indentations.