Add error on imports inside class definitions

[IanLee1521]

Opening a new issue for this based on discussion from @ncoghlan on issue #304 .

Flagging module level imports that don't occur at the start of the file and imports inside class definitions as errors seem like reasonable additions to me. The latter is technically going beyond the text of the PEP itself, but I'd be incredibly surprised if anyone complained about you flagging it as an error - while imports at class scope are permitted by the language, deliberately adding a module as a class attribute would just be weird.

[jayvdb]

I would love to see more granularity for import style errors. We've had to disable the new E402 as there are too many complex import hierarchy issues to resolve before we could enable it, and other style problems have got a higher priority. But if there were different error codes for the wierder imports, such as within classes like this issue, I'd expect we'd tackle those immediately (even if it meant using import/imp/etc with a TODO, in which case they become trackable) so that new similar naughtiness cant be introduced to the code base.