BUG: DatetimeIndex.unique numpy 1.6 repr #2563
#2563

Boxing + dtype=object hash table pass (vs. int64) --> 300x slowdown. we can make this into a vbenchmark

```
In [48]: rng = date_range('1/1/2000', periods=10000, freq='T')

In [49]: arr = rng.repeat(10)

In [50]: timeit arr.unique()
1000 loops, best of 3: 1.11 ms per loop

In [51]: timeit arr.asobject.unique()
1 loops, best of 3: 320 ms per loop
```
