Excel export of the list view

Hi guys, 

As I really need an Excel export functionality, I decided to create this pull-request. I've added the needed functionality using the coding style used in the Bundle (or at least, I hope). 

For this functionality I've extended the ListAction, to create a simple coupling between the list results and the export. I've added everything for all three ORM: however, I only got to test Doctrine. The others should work just fine as I've only added one method (`getResults()`). 

Can you guys give me feedback? I will update this pull request with the new commits whenever needed.