Add support for PSR-3

[neomerx]

Add optional logging (debug feature) for PSR-3

[neomerx]

In all major lib parts logs could be written as following

// no need to check for null
$this->log->debug(...);

As the lib is extremely tested I don't feel any specific logs are needed at the moment. For debugging purposes logs could be turn on by replacing Encoder::instance(...) with $this->createLoggedEncoder(...) which will save logs in root folder. Logs are based on monolog so they are highly configurable.

External users might use own logs as following

$log     = ...;
$factory = new Factory();
$factory->setLogger($log);

$container = $factory->createContainer($schemas);
$encoder   = $factory->createEncoder($container, $encodeOptions);

$encoder->encodeData(...);

Logs tested and working. Don't see anything to add at the moment.