#108 : BaseAdminExtension refactor

 - rename BaseAdminExtension to Admin\Entension\BlockCacheExtension
 - remove the enabled field mapping and plug methods to publishable to fit BlockInterface
 - create a new Cache form group for admin form
 - plug every service to isPublishable to fit the publishWorkflow

to enabled this extension you need to add this in your config file :

    sonata_admin:
        extensions:
            cmf_block.admin_extension.cache:
                implements:
                    - Sonata\BlockBundle\Model\BlockInterface
#109 :
- rename cache form group to metadata to be generic
- rollback using isPublishable in block service to fit BlockInterface
Merge pull request #115 from nicolas-bastien/Issue_108_refactoring_extension

#108 : BaseAdminExtension refactor