Jobqueue logger

@scottrabin 
Add Wikia\Logger\Loggable trait, and use it in BaseTask
Suggestion: a generalized `log` function that also accepts a level, with the rest of the functions being convenience wrappers on top

```php
public function log($level, $message, $context=[]) {
    call_user_func(
        array(WikiaLogger::instance(), $level),
        $message,
        $this->mergeLoggerContext($context)
    );
}
```

(maybe with some guards on the value of `$level` and definitely with some defined constants)

There are a number of calls in this pull request that do `if ($something) $this->log($msg, $ctx); else $this->warning($msg, $ctx);`, and it might be nicer to have just one logging call with a level parameter.

It's just a suggestion; if you see some issue with this, we should discuss it.