[RFC] Separation ListElement from CRUD

This PR is related to #71. 

My main goal is to remove CRUD as a single "admin element" from FSiAdminBundle. I dont like it because it  have too many responsibilities. I believe there is a way to provide each part of current CRUD element as a single element and it simple to use.
This PR is about ListElement (probably the easiest one).  
Following diagram represents ListElement inheritance tree:

```
+––––––––––––––––––––––––+          +––––––––––––––––––––––––––+
| AdminElement           +––––––––––+  Element Interface       |
+––––––––––+–––––––––––––+          +––––––––––––––––––––––––––+
           |                                                    
           |                                                    
           |                                                    
           |                                                    
+––––––––––+–––––––––––––+          +––––––––––––––––––––––––––+
| BaseListElement        +––––––––––+ ListElement Interface    |
+––––––––––+–––––––––––––+          +––––––––––––––––––––––––––+
           |                                                    
           |                                                    
           |                                                    
           |                                                    
+––––––––––+–––––––––––––+          +––––––––––––––––––––––––––+
| DoctrineListElement    +––––––––––+ DoctrineElement Interface|
+––––––––––––––––––––––––+          +––––––––––––––––––––––––––+

```

So as you can see first is AdminElement that imlements Element interface, next we have BaseListElement that implements ListElement interface and DoctrineListElement that implements DoctrinElementInterface. 
This is how I'm going to create other EditElement & CreateElement

But I have few problems with this approach. 

* How easy bind ListElement with CreateElement/EditElement  
* How not add end user to much job with building elements and registering them as services
* How to solve "Delete" 

In next few days I'm going to try few different ideas and choose best solution but If anybody is willing to help me I would be thankeful for any thoughts. Do not hasitate to ask any questions about this PR, there are not stupid questions :) 

*This is only a draft, it does not have any tests because I will rewrite it, I'm just looking for the best approach at the moment.*