Added two methods to access cache in array style

For v.2.1
**Example**

to get some value from an array in cache use this method:      

```php 
_get("arrayName[key0][key1][key2]");
```

to set a value:
```php 
_set("arrayName[key0][key1][key2]" , $new_value);
_set("arrayName[key0][key1][key2]" , $new_value, 300);
```

I'm sure those methods' names aren't the best, so it would be great to find another ones.