Make Browser second constructor argument, de-emphasize default loop

[clue]

This changeset simplifies usage by making the Browser the second argument and the LoopInterface the last argument to de-emphasize the default loop. Most consumers of this package are not expected to use either parameter, so this should not affect them. This is a BC break for more advanced use cases that do require either argument.

Together with #22 and #23, this means we can now fully rely on the default loop and no longer need to skip any arguments with null values.

// old
$es = new Clue\React\EventSource\EventSource('https://example.com/stream.php');
$es = new Clue\React\EventSource\EventSource('https://example.com/stream.php', null, $browser);
$es = new Clue\React\EventSource\EventSource('https://example.com/stream.php', $loop, $browser);

// new
$es = new Clue\React\EventSource\EventSource('https://example.com/stream.php');
$es = new Clue\React\EventSource\EventSource('https://example.com/stream.php', $browser);
$es = new Clue\React\EventSource\EventSource('https://example.com/stream.php', $browser, $loop);

Refs #10/#17 to simplify passing a custom Browser object which could potentially hold custom default headers.

Builds on top of #22, #23, #4 and possibly others