Updated transformation with regards to the latest updates in the develop branch in Imbo

This PR updates the image transformations to work with the latest changes in the develop branch of Imbo, where image transformations have been changed to be event listeners.

I have also added a config file that can be used for enabling these image transformations, as mentioned in the [imbo docs](http://docs.imbo-project.org/en/develop/installation/installation.html#using-composer).

A simple test suite have been added as well.