dtrace: fix generation of v8 constants on freebsd

Every constant is certainly 4 bytes now, but freebsd's objdump utility
prints out odd byte sequences (5-bytes, 6-bytes and even 9-bytes long)
for v8's data section. We can safely ignore all upper bytes, because all
constants that we're using are just `int`s. Since on all supported
platforms `int` is 32bit long (and anyway v8's constants are 32bit too),
we ignore all higher bits if they were read.
gyp: fix build with dtrace support on FreeBSD

Fix undefined reference to `gelf_getsym`... and other undefined symbols
from libelf, by adding `-lelf` to linker options on FreeBSD.
doc: Fix syntax in cluster example code
deps: upgrade libuv to 7841f77
deps: upgrade libuv to e4d8cba
http: close connection on 304 and chunked encoding

Force the connection to close when the response is a 304 Not Modified
and the user has set a "Transfer-Encoding: chunked" header.

RFC 2616 mandates that 304 responses MUST NOT have a body but node.js
used to send out a zero chunk anyway to accommodate clients that don't
have special handling for 304 responses.

It was pointed out that this might confuse reverse proxies to the point
of creating security liabilities, so suppress the zero chunk and force
the connection to close.
buffer: remove float write range checks

Removed range checks when writing float values, and removed a few
includes and defines. Also updated api docs to reflect that invalid 32
bit float is an unspecified behavior.
build: Add some gyp artifacts to .gitignore