Add an ``edit`` button in sonata_type_model form

Add an `edit` button for sonata-type-model form with `many-to-one` relations.
Button is rewriten and enabled when an option is selected (or already selected)

Edit button uses the traditional modal in create mode.

![edit_hover](https://cloud.githubusercontent.com/assets/5037520/4607866/5dd609ae-5263-11e4-81df-1ed45d612538.png)

I don't know if this approch is not too intrusive (loocking for `select` tags, find an `edit` button to rewrite, change this one at any change on select, ...). Any advice would be appreciated :-)

The url generation can be quite tricky if you have a composite PK.