BUG: pd.unique should respect datetime64 and timedelta64 dtypes (GH9431)
To fix [GH9431](https://github.com/pydata/pandas/issues/9431).

Previously `pd.unique` would return an array of  `object` dtype when passed a 1D array, Series or Index with a `datetime64` or `timedelta64` dtype. This PR should remedy that behaviour.
