Buglet in Dataframe.append with no index
This works as expected:

```
    df = DataFrame(index=[0], columns=['A', 'B', 'C'])
    df3 = DataFrame(index=[1, 2], columns=['A', 'B'])
    df5 = df.append(df3)

    expected = DataFrame({'A': [nan, nan, nan],
                          'B': [nan, nan, nan],
                          'C': [nan, nan, nan]})
```

This doesn't:

```
    df = DataFrame(columns=['A', 'B', 'C'])
    df3 = DataFrame(index=[0, 1], columns=['A', 'B'])
    df5 = df.append(df3)

    expected = DataFrame({'A': [nan, nan],
                          'B': [nan, nan],
                          'C': [nan, nan]})
```
