Integers as labels in docs
Is the current behavior a regression/bug or are the docs incorrect?

This [1] suggests that the following should raise an error. It doesn't for me on f1d35ee nor is its behavior obvious.

```
randn = np.random.randn
df = pd.DataFrame(randn(5,5))
df.ix[-2:]
```

[1] http://pandas.pydata.org/pandas-docs/dev/gotchas.html#integer-indexing
