Add SMW JobQueue statistics

Allow some insights about SMW related jobs and display them via `Special:Statistics`. `$smwgShowJobQueueStatistics` can be used to disable the display.

[0] http://wikimedia.7.x6.nabble.com/Re-Job-queue-affecting-semantic-queries-in-MW-1-22-tp5029433.html
In this case I have to map the method name `getUpdateJobCount` to a message key, currently `UPDATEJOB` is mapped to the message key.

Rename `getStatistics` to `getJobQueueCount`

```
+	private $jobQueueMessageMapper = array(
 +		'UPDATEJOB'   => 'smw-statistics-jobqueue-update-count',
 +		'REFRESHJOB'  => 'smw-statistics-jobqueue-refresh-count',
 +		'DELETEJOB'   => 'smw-statistics-jobqueue-delete-count'
 +	);
```