DatetimeIndex very slow repr performance
Hi folks, it's been a while. I would like to draw your attention to:

```
dr = pd.date_range('20000101', freq='D', periods=100000)
%time dr._is_dates_only
```

This takes about 500ms on my machine. Scales linearly with the size of the index (I interrupted a repr on a 5mm-length index and discovered this). Probably a good idea to fix this for 0.15.0 (this is on master)
