Add main Connector facade

[clue]

This PR adds a new Connector class that acts as a facade for all underlying connectors, which are now marked as "advanced usage", but continue to work unchanged.

The Connector class now makes it trivially easy to create plaintext TCP/IP, secure TLS and Unix domain socket connection streams like this:

$connector = new Connector($loop);

$connector->connect('tls://google.com:443')->then(function (ConnectionInterface $conn) {
    $conn->write('…');
});

Resolves / closes #38

[jsor]

I'd suggest the following exception message:

return Promise\reject(new RuntimeException(
    sprintf(
        'No connector registered for URI scheme %s',
        $scheme
    )
));

[jsor]

Suggestion: if no scheme is explicitely given and port is 443, use tls connector.

[jsor]

More magic:

• google.com with 'tcp' => false -> Use tls
• google.com:443 with 'tls' => false -> Use tcp

[clue]

Both suggestions make sense in higher-level protocols (e.g. HTTP/HTTPS), but I'm hesitant to add them in the low-level network protocols (TCP/IP etc.) here.

I believe this is best left up to consumers of this package (such as react/http-client), but I guess it makes sense to add some kind of support for these kind of things in future versions 👍

[clue]

Thanks for the suggestion, updated and squashed to use improved error message