Jobqueue logger

@scottrabin 
Add Wikia\Logger\Loggable trait, and use it in BaseTask
I added typehints, don't know why I didn't do that to begin with.

As for a generalized log function, I don't think it's actually too useful, since you will also have to change the message going into the logged function to avoid confusion such as "Why is this message sometimes level warning, but sometimes level info?" So your code will end up looking something like this:

```php
$level = LOG_INFO;
$message = 'i did some stuff';
// do something
if ($somethingIsBroken) {
  $level = LOG_ERR;
  $message = 'i failed to do some stuff';
}
$this->log($level, $message, $context);
``` 

Seems harder than writing

```php
// do something
if ($somethingIsBroken) {
  $this->error('i failed to do something', $context);
} else {
  $this->info('i did something', $context);
}
```