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.

- [x] Fix and move `upload` type and rename to `collection_upload`
- [x] Fix and move `single_upload` type
- [x] Move `collection` fieldset and table widgets into new form types `collection_fieldset` and `collection_table`
- [x] Add `color_picker` form type
- [x] Add Select2 form types: `select2_choice`, `select2_language`, `select2_country`, `select2_timezone`, `select2_locale`, `select2_entity`, `select2_document`, `select2_model`
- [x] Remove EntityPicker widget (it will no longer be maintained, use `select2_entity`, `select2_document` or `select2_model` (depending on your ORM) instead
- [ ] Move `double_list` type
- [ ] Add date form types: `date_picker`, `datetime_picker`, `daterange_picker`