fs: implemented WriteStream#writev

Streams with writev allow many buffers to be pushed to underlying OS
APIs in one batch, in this case improving write throughput by an order
of magnitude. This is especially noticeable when writing many (small)
buffers.

PR-URL: https://github.com/nodejs/node/pull/2167


Reviewed-By: Trevor Norris <trev.norris@gmail.com>