Enable .drop(...) to work with MultiIndex
So we could do like df.drop(['foo']) on this:

```
            A       B       C        foo
foo  one   -0.659   0.7982  0.2346   bar
     two    2.266  -1.157   0.04916  bar
     three -1.786   2.029   0.4018   bar
bar  one    0.4682  2.42   -1.073    bar
     two   -0.1343 -0.4565 -0.7504   bar
baz  two   -1.387  -0.9626 -0.9333   bar
     three  1.277  -2.475   0.9841   bar
qux  one   -0.633  -0.9965  0.6696   bar
     two    1.359  -0.6917 -0.5386   bar
     three  0.5482  0.2563  0.6752   bar
```

and get this:

```
            A       B       C        foo
bar  one    0.4682  2.42   -1.073    bar
     two   -0.1343 -0.4565 -0.7504   bar
baz  two   -1.387  -0.9626 -0.9333   bar
     three  1.277  -2.475   0.9841   bar
qux  one   -0.633  -0.9965  0.6696   bar
     two    1.359  -0.6917 -0.5386   bar
     three  0.5482  0.2563  0.6752   bar
```
