Replace listen() call with URIs passed to constructor

[clue]

The current socket API exhibits a temporal dependence, i.e. it's not safe to call listen() or shutdown() multiple times on the same instance and call order is actually significant. (see #19)

This PR replaces the listen($port, $host) API with URIs passed to the constructor.
This means the Server now always represents a listening Socket (which may
possibly be in a closed state).

• URIs are literally the only way to provide a consistent addressing scheme throughout React's components, see Consistent addressing scheme reactphp#199
• Not all protocols know the concept of a "port", for example the future UnixServer currently would have an unused $port parameter (Add support for Unix domain sockets (UDS) #17)
• The Server class accepts a single port parameter instead of a full URI and assumes a localhost socket in this case for better compatibility with previous releases.

This is obviously a BC break from a consumer perspective, but for the most part this is easily patched like this:

- $server = new Server($loop);
- $server->listen($port);
+ $server = new Server($port, $loop);

Also, empirical evidence suggests many places simply consume a ServerInterface in order to react to the connection event - this very common API is not affected.

This means that the Server class now starts listening immediately and a consumer can no longer defer the listen() call. I personally consider this a good thing (poka yoke) as I know many people have stumbled over this. Also, this API is now somewhat similar to Node's net.createServer() which also starts up a listening server (vs. the older explicit server.listen() call).

If you want to review this PR and don't want to mess with all these details, I'd suggest taking a look at the examples (and their minimal diff). The previous CLI accepted merely an optional port, the new CLI also accepts full listening addresses, for example like this:

$ php examples/02-chat.php 0.0.0.0:8080

Closes #19
Supersedes / closes #6, refs reactphp-legacy/socket-client#74, reactphp/reactphp#199 and #34

[jsor]

Minor nitpicking (and personal preference): I'd like to see an early return here.

if (!is_resource($this->master)) {
    return;
}

$this->loop->removeStream($this->master);
fclose($this->master);
$this->removeAllListeners();