Allow assets dicts to be called like attrs

[ojustino]

Allow keys of a dictionary to also be called like class attributes:

```
    >>> regions = {'Milotic': 'Hoenn'}
    >>> regions['Milotic']
    Hoenn
    >>> regions.Milotic
    AttributeError: 'dict' object has no attribute 'Milotic'
    
    >>> regions = AttrDict({'Milotic': 'Hoenn'})
    >>> regions['Milotic']  # standard dict behavior
    Hoenn
    >>> regions.Milotic  # behavior enabled by AttrDict
    Hoenn
```

Also handles setting and deleting keys/attrs. Nested dicts are also
converted to AttrDict instances. Less-than-free-range code.