PSR-3 SMW\Log\LoggerInterface

Replace wfDebug calls with a MW specific LoggerInterface [1] implementation.

## Usage
A class that needs access to a logger should implement the LoggerAware
interface. The setLogger invocation should be handled by the DI or
ServiceFactory, a NullLogger is ought to be set as default.

[1] https://github.com/php-fig/log/blob/master/Psr/Log/LoggerInterface.php