benchmark: Add a test to measure Buffer.slice perf

Buffer.slice can be expensive. One regression was reported by https://github.com/joyent/node/issues/7633. The method should be benchmarked.
doc: Add a README for benchmark tests

The README.md documents how to run node core benchmark tests and
how to write new tests.
net: Ensure consistent binding to IPV6 if address is absent

See https://github.com/joyent/node/issues/7675
net.server.listen() behaves inconsistently depending on whether the port
number is provided.

1. port === 0 && host == '' (i.e. false-y), node creates an AF_INET
socket but does not call bind().

2. port > 0 && host == '', node creates an AF_INET6 socket and calls
bind().

The fix makes 1 consistent with 2.

Signed-off-by: Fedor Indutny <fedor@indutny.com>