Fixes support for nested loggers when using multiple Logger instances

[lux]

This ought to make it possible to support nested loggers with multiple Logger instances, fixing #49.

Usage is slightly different, where now instead of each handler being a static ::init() call, we instantiate the handler via new Handler\LevelBuffer() at the first level and Analog should take care of the rest.

<?php

require 'vendor/autoload.php';

use Analog\Logger;
use Analog\Handler;

$log1 = '';
$log2 = '';

$logger1 = new Logger ();
$logger1->handler (new Analog\Handler\LevelName (
	Handler\Variable::init ($log1)
));

$logger2 = new Logger ();
$logger2->handler (new Analog\Handler\LevelBuffer (
	Handler\Variable::init ($log2),
	Analog::ERROR
));

$logger1->debug ('Log 1.1');
$logger2->debug ('Log 2.1');

$logger1->debug ('Log 1.2');
$logger2->debug ('Log 2.2');

$logger1->debug ('Log 1.3');
$logger2->error ('Log 2.3');

echo $log1;
echo '---' . PHP_EOL;
echo $log2;