Series.unique converts uint64 to int64 (with overflow)
```
>>> import pandas as pd
>>> import numpy as np
>>> s = pd.Series([1,2,2**63], dtype=np.dtype('uint64'))
>>> s
0                      1
1                      2
2    9223372036854775808
dtype: uint64
>>> s.unique()
array([                   1,                    2, -9223372036854775808])

```

Series.unique should preserve whatever type the values are.  But instead it changes from uint64 to int64.

INSTALLED VERSIONS
------------------
...
python: 3.4.3.final.0
python-bits: 64
...

...
pandas: 0.17.1
numpy: 1.10.4
...