Implemented logging.

Added class Logger, which manages set of logging plugins.
By default, only one logging plugin is loaded, DummyLogger, which does
nothing.
Implemented two other plugins: FileLogger and XMPPLogger.
First one dumps messages to a file.
Second one sends messages to Node.js server (https://bitbucket.org/element_doo/dsl-logging-server) to send them over XMPP protocol.
Added logging of REST Http requests.

Example usage.
Index.php:
    require_once __DIR__.'/../platform/Bootstrap.php';
    use NGS\Logger;
    use NGS\FileLogger;
    use NGS\XmppLogger;

    $logger = new Logger(array(
    	new XMPPLogger('http://localhost:9989'),
    	new FileLogger(__DIR__.'/logs.txt')
    ), Logger::INFO);
    Logger::instance($logger);
../Requirements.php
bye bye dummylogger.
Merge remote-tracking branch 'robi/logging' into logging

Conflicts:
	NGS/Client/HttpRequest.php
	NGS/Logger/Logger.php
	NGS/Logger/XMPPLogger.php
Backup commit. Sve radi, treba redizajnirat.
Logging implemented.

Loggers are now implementing PSR-3 LoggerInterface and using PSR-3's
LogLevels.
Added PsrLoggerBridge which connects LoggingInterfaces with RestHttp's
logging events.
Removed XMPPLogger