Add Entity with Multiple bundles base class for listing

This class allows a query on a specific entity, with explicitly defined bundles and the resource that should be used in order to render them.
This can be used for example with the "Message" module, where you would like to get a list of predefined messages form the server.

![restful-multiple-bundles](https://cloud.githubusercontent.com/assets/6566363/2899443/452002ae-d5b6-11e3-87c3-951c10d0c2d9.png)