down methods implemented on migration classes and drop task created

config/orientdb.yml

lib/orient_db/migration.rb

lib/orient_db/migrations.rb

lib/orient_db/migrations/create_attribute.rb

lib/orient_db/migrations/create_belongs_to.rb

lib/orient_db/migrations/create_collection.rb

lib/orient_db/migrations/create_comment.rb

lib/orient_db/migrations/create_commented_by.rb

lib/orient_db/migrations/create_comments.rb

lib/orient_db/migrations/create_has_attr.rb

lib/orient_db/migrations/create_institution.rb

lib/orient_db/migrations/create_is_about.rb

lib/orient_db/migrations/create_learning_object.rb

lib/orient_db/migrations/create_like.rb

lib/orient_db/migrations/create_main_page.rb

lib/orient_db/migrations/create_object.rb

lib/orient_db/migrations/create_published_by.rb

lib/orient_db/migrations/create_subject.rb

lib/orient_db/migrations/create_user.rb

lib/tasks/orientdb.rake