stub for Chef::DataBagItem #save method.

I want to have ability to save DataBagItem inside my chef scripts. Something like this: 
```ruby
redis_master_config = Chef::DataBagItem.load('redis', 'master')
redis_master_config['address'] = @master['address']
redis_master_config.save
```
PR is my quick work around. 

@thbishop what do you think about it ?