[Soc2014] Official meta API

Also, am I right that of the flags `many_to_many`, `many_to_one`, `one_to_many`, and `one_to_one`, only one of them should be `True` for any given field class? And none of them set to `True` if it's not a relation field? If so, it seems to me that `FieldFlagsMixin` should default them all to `False` so that subclasses only have to define one of them, not all four.