Override __dir__ to allow for tab completion in ipython and easier inspection

The common thing to do if you override `__getattr__` is to also override `__dir__` like so:

``` python
def __dir__(self):
    # you may also want to filter the keys to only include the strings
    return sorted(set(super(AttrDict, self).__dir__()) + set(self.keys()))
```

This would make it much easier to use attrdict in IPython and other IDEs as you would gain auto-completion.
