Implement html field type

Currently, there is no way to display safe html in a sonata list.


![ckeditor](https://cloud.githubusercontent.com/assets/2140469/4921603/7141c64e-650b-11e4-94e3-cbfbcfa33140.png)
![truncate](https://cloud.githubusercontent.com/assets/2140469/4921605/71493a96-650b-11e4-8926-95dbb565985b.png)
![raw](https://cloud.githubusercontent.com/assets/2140469/4921604/7142be28-650b-11e4-9a6b-42cbb0b70340.png)