Implemented logging.

Added class Logger, which manages set of logging plugins.
By default, logging is disabled.

Implemented two logging 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);