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:

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

Resolves / closes#38

[jsor]

I'd suggest the following exception message:

returnPromise\reject(newRuntimeException(
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