ENH: _dir_additions returns also the first level of a MultiIndex
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff``
 - [x] whatsnew entry

Assuming a DataFrame with MultiIndex columns like this:
```python
>> df
    foo   bar      
          foo   bar
0  0.32  0.15  0.45
1  0.62  0.73  0.36
2  0.21  0.68  0.10
3  0.05  0.36  0.90
```

one can access to `df['foo']` and `df['bar']` with the shortcuts `df.foo` and `df.bar`,
but one don't benefit from autocompletion because 'foo' and 'bar' are not listed in `dir(df)`. 

```python
>> 'bar' in dir(df)
False
```

This PR extends `df._dir_additions` so that the first level of a MultiIndex is listed in `dir(df)`.

```python
>> 'bar' in dir(df)
True
```
