BUG: Fix long index repr #2319
``` python
In [2]: pd.Index(np.arange(10000))
Out[2]: Int64Index([0, 1, 2, ..., 9997, 9998, 9999], dtype=int64)
```

broken by 95678ebf.
configurable via `np.set_printoptions (threshold)`, like the original version which used np.ndarray.**repr**.

closes #2319
