Monkey patch Mash to allow "dot" notation of resource accessors

Ohai 7 does not allow users to call attributes by method names. Previously in Ohai 6, one could do something like:

```ruby
Ohai.kernel.machine
```

In Ohai 7, you must use Ohai as a hash:

```ruby
Ohai['kernel']['machine']
```

This commit restores the functionality presented with the previous version of Ohai, while adding a deprecation message that you should not use the "dot" notation in the future.