FileReader should be a Producer

[`riko.bado.io.FileReader`](https://github.com/nerevu/riko/blob/0a3cab171b0e4ccdd2e47ecceef44d837f1747c6/riko/bado/io.py#L40) should be a [`push_ producer`](https://twistedmatrix.com/documents/current/core/howto/producers.html#push-producers).  This will be easier to implement and conform to most Consumer's expectations.