ENH: NDFrame.equals
seems we need a public version of `tm.assert_frame_equal`

maybe `equals` on generic NDFrame objects

http://stackoverflow.com/questions/19322506/pandas-nan-comparison

seems good on the values comparison (should compare axes/types as well of course)

```
((df1 == df2) | ((df != df) & (df2 != df2))).values.all()
```
