Introduces the Statisticser interface which ResultIterators may
implement.
The HTTP implementation uses HTTP trailers to preserve the statistics.
This way we do not need to have all encoders and decoders support
statistics.
This modifies the `MultiResultDecoder` interface to accept an
`io.ReadCloser` so the `ResultIterator` can close the `io.Reader`
instead of doing it through a defer call. It then makes it so the
`Cancel()` method will close the reader or the reader will be
automatically closed when `More()` returns false.
Nathaniel Cook
Jonathan A. Sternberg