Bugfix: mysql.query.stream returns a stream.Readable.

See mysql docs at https://github.com/felixge/node-mysql#piping-results-with-streams2

Both the typings and the tests had a bug. According to the docs, IQuery.stream() returns a stream.Readable. In the typings, it returned a custom interface with a wrong pipe() method signature.

In order to import 'streams', I had to refactor the typings into an external module declaration. I don't think this is a problem since you cannot use node-mysql without node.