Fix form horizontal inheritance and add "form-horizontal" to the root fo...

https://github.com/phiamo/MopaBootstrapBundle/issues/813
https://github.com/phiamo/MopaBootstrapBundle/issues/801
https://github.com/phiamo/MopaBootstrapBundle/issues/775

This will make it so the `horizontal` view variable is inherited from the element's parent. So if you want to non-horizontal a whole form you can do that, or if you want a specific sub-section to all be non-horizontal you can do that too. 

This also adds form-horizontal to the form itself so you can now use `form_start` / `form` and not have to use setDefaultOptions to add a class name.