Metadata marker, integrating with QA contact plugin

Easy way how to add metadata to the tests. Then plugins can read them or tests can read them. Every test node receives a `._metadata` dictionary modified that it allows picking values up using object variable access (`.key`). It returns none if the value is not there so the checks are simple. Also a special fixture called `meta` is provided that is a shortcut for `request.node._metadata`.
`return self.get(attr)`?