[Soc2014] Official meta API

I'd like to see a better analysis than "backends is heavily tested." The comment you added gives no indication why M2M fields should be filtered out. I think more research is needed. I'm also uncomfortable using `has_many_values` as a proxy for `ManyToManyField`. After all, the docs for `has_many_values` say, "This does not necessarily need to be a ManyToMany field, but can also be a `CommaSeparatedIntegerField` for example." While I wouldn't expect `CommaSeparatedIntegerField` to appear in `related_objects`, if we are making `has_many_values` a public API, are we sure that this particular bit of code isn't relevant to custom fields that might set `has_many_values` to `True`?