update the stream module to use only the non-flowing (0.10+) stream API

It's slightly more efficient, and most implementations of the old API are
broken anyway, because nobody else uses it.

Also modified ReadableStringStream / WritableStringStream to use new API.
The new API requires implementors support binary, so now there is also
ReadableStream and WritableStream which can work with binary or strings
(if an encoding is set). The String versions now just wrap the buffer
versions, defaulting to UTF-8.

ReadableString no longer supports the `paused` argument. That shouldn't
affect anyone using the modern stream API, but may affect some
code expecting nodejs < 0.10 behaviour.
tar: wrap old style streams