API: Add DataFrame.droplevel

As a way to make dropping an index level possible in an method chain.

```python
>>> df = pd.DataFrame(np.arange(12).reshape(4, 3), columns=['a', 'b', 'c']).set_index(['a', 'b'])
>>> df.droplevel(0)
```

roughly equivalent to 

```python
df2 = df.copy()
df2.index = df.index.droplevel(0)
df2
```

cc @twiecki 