Forms and menu refactoring

This PR removes all form types from Admingenerator project. The forms have been moved to
[avocode/FormExtensions](https://github.com/avocode/FormExtensions) bundle and will be 
developed there. This loosens Admingenerator's dependency and makes it easier to use
diffrent forms.

This PR also changes the way admingenerator deals with menu. Creating and customizing menus will be easier now. For details visit the [cookbook](https://github.com/symfony2admingenerator/AdmingeneratorGeneratorBundle/blob/master/Resources/doc/cookbook/menu.md) entry.