Added item level calculations

Added item level total, average and slot array

```php
$Character->getItemLevelArray();
$Character->getItemLevelTotal();
$Character->getItemLevelAverage();

[item_level_array] => Array
	(
		[main] => 180
		[main2] => 0
		[shield] => 0
		[head] => 90
		[body] => 100
		[hands] => 90
		[waist] => 90
		[legs] => 90
		[feet] => 90
		[necklace] => 90
		[earrings] => 90
		[bracelets] => 90
		[ring] => 90
		[ring2] => 90
	)

[item_level_total] => 1180
[item_level_average] => 90
```