make the wrapping of addons configurable

When in an horizontal form, you probably do not want to have fields with
addons wrapped in a form-group div, since the negative margins of this
div will make the field looks larger than the other fields.
infer config from sonata_admin configuration

If someone configures sonata_admin to use horizontal form, they probably
want to do the same for sonata_core, don't they
test configuration
test dependency injection
Merge pull request #108 from greg0ire/wrapping_configurability

make the wrapping of addons configurable