stream: simpler stream constructon

Adds simplified constructor pattern, allowing users
to provide "read", "write", "transform", "flush", and
"writev" functions as stream options in lieu of subclassing.

Semver: minor
PR-URL: https://github.com/iojs/io.js/pull/697
Fixes: https://github.com/iojs/readable-stream/issues/102


Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>