unification of meta-attributes handling: * remove has_format, has_text_encoding, format, text_encoding methods on Entity * remove rich_text_fields method on EntitySchema * use instead has_metadata / meta_attribute on EntitySchema (from yams) and attribute_metadata on Entity

# Instructions here will be read before reading the schema
# You could create your own groups here, like in :
#   add_entity('EGroup', name=u'mygroup')