Add the Admin extension that enables editing HTML attributes

Contains an interface that encapsulates the attributes related methods for the menuNode.
Relies on burgov/key-value-form-bundle for the admin.
we silently bail out only if use admin is set to `auto`. but if its set to true but no sonata admin is available, we could indeed throw an explicit exception instead of just going on and then have some missing class (but thats still a bit better than missing services).

@mkoosej what i mean is: use_sonata_admin could be false. but SonataAdminBundle is a dependency for the admin extension, you implement an interface. (but not SonataDoctrintePHPCRAdminBundle). so the two options edit_attributes (we should call it sonata_extension_menu_options) and use_sonata_admin should be handled completely independant of each other. for your new option, you check for burgov and the base sonata bundle.