http2: introducing HTTP/2

At long last: The initial *experimental* implementation of HTTP/2.

This is an accumulation of the work that has been done in the nodejs/http2
repository, squashed down to a couple of commits. The original commit
history has been preserved in the nodejs/http2 repository.

This PR introduces the nghttp2 C library as a new dependency. This library
provides the majority of the HTTP/2 protocol implementation, with the rest
of the code here providing the mapping of the library into a usable JS API.

Within src, a handful of new node_http2_*.c and node_http2_*.h files are
introduced. These provide the internal mechanisms that interface with nghttp
and define the `process.binding('http2')` interface.

The JS API is defined within `internal/http2/*.js`.

There are two APIs provided: Core and Compat.

The Core API is HTTP/2 specific and is designed to be as minimal and as
efficient as possible.

The Compat API is intended to be as close to the existing HTTP/1 API as
possible, with some exceptions.

Tests, documentation and initial benchmarks are included.

The `http2` module is gated by a new `--expose-http2` command line flag.
When used, `require('http2')` will be exposed to users. Note that there
is an existing `http2` module on npm that would be impacted by the introduction
of this module, which is the main reason for gating this behind a flag.

When using `require('http2')` the first time, a process warning will be
emitted indicating that an experimental feature is being used.

To run the benchmarks, the `h2load` tool (part of the nghttp project) is
required: `./node benchmarks/http2/simple.js benchmarker=h2load`. Only
two benchmarks are currently available.

Additional configuration options to enable verbose debugging are provided:

$ ./configure --debug-http2 --debug-nghttp2
$ NODE_DEBUG=http2 ./node

The `--debug-http2` configuration option enables verbose debug statements
from the `src/node_http2_*` files. The `--debug-nghttp2` enables the nghttp
library's own verbose debug output. The `NODE_DEBUG=http2` enables JS-level
debug output.

The following illustrates as simple HTTP/2 server and client interaction:

(The HTTP/2 client and server support both plain text and TLS connections)

```jt client = http2.connect('http://localhost:80');
const req = client.request({ ':path': '/some/path' });
req.on('data', (chunk) => { /* do something with the data */ });
req.on('end', () => {

// Plain text (non-TLS server)
const server = http2.createServer();
server.on('stream', (stream, requestHeaders) => {
  stream.respond({ ':status': 200 });
  stream.write('hello ');

const http2 = require('http2');
const client = http2.connect('http://localhost');


Author: Anna Henningsen <anna@addaleax.net>
Author: Colin Ihrig <cjihrig@gmail.com>
Author: Daniel Bevenius <daniel.bevenius@gmail.com>
Author: James M Snell <jasnell@gmail.com>
Author: Jun Mukai
Author: Kelvin Jin
Author: Matteo Collina <matteo.collina@gmail.com>
Author: Robert Kowalski <rok@kowalski.gd>
Author: Santiago Gimeno <santiago.gimeno@gmail.com>
Author: Sebastiaan Deckers <sebdeckers83@gmail.com>
Author: Yosuke Furukawa <yosuke.furukawa@gmail.com>



http2: refactor trailers API

Rather than using the `'fetchTrailers'` event to collect trailers,
a new `getTrailers` callback option is supported. If not set, the
internals will skip calling out for trailers at all. Expands the
test to make sure trailers work from the client side also.

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