Method for df
See http://stackoverflow.com/questions/11941492/selecting-rows-from-a-pandas-dataframe-with-a-compound-hierarchical-index

let's say I want to filter a DataFrame such that level[k] only contains the values x, y or z.
A vectorized possibility is:

```
df[np.in1d(df.index.labels[k], match([x, y, z], df.index.levels[k]))]

```

but I guess it would be nice to have some pandas method wrapping it or implementing it more efficiently.
