Added universal 'get_data_bag_item' helper 

It might be useful for library cookbook developers. Let's assume, that you are expecting to get the data bag item, but you really don't (and you can't) know is it encrypted or not. Just use this pretty helper:
```
item = get_data_bag_item('data_bag_name', 'item_name')
```
The item will be automatically decrypted if needed (like as `Chef::EncryptedDataBagItem.load`). Otherwise, the raw data will be returned (like as `Chef:: DataBagItem.load`)

P.s. Any suggestions are welcome.