unique() broken for datetime64[ns] columns
I have a data frame containing a column with timestamps using the numpy datetime data type. The unique() function on that column returns obviously invalid data:

``` python
cues[(cues.component == '/usr/bin/naoqi-bin') & (cues.thread_id == 0)].start_time
Out[50]: 
time
2013-02-14 10:11:13.480284   2013-02-14 10:09:34.640000
2013-02-14 10:11:13.728758   2013-02-14 10:09:34.640000
2013-02-14 10:11:13.979455   2013-02-14 10:09:34.640000
2013-02-14 10:11:14.240253   2013-02-14 10:09:34.640000
2013-02-14 10:11:14.484151   2013-02-14 10:09:34.640000
2013-02-14 10:11:14.718189   2013-02-14 10:09:34.640000
2013-02-14 10:11:14.969820   2013-02-14 10:09:34.640000
2013-02-14 10:11:15.237294   2013-02-14 10:09:34.640000
2013-02-14 10:11:15.471394   2013-02-14 10:09:34.640000
2013-02-14 10:11:15.729639   2013-02-14 10:09:34.640000
2013-02-14 10:11:15.980261   2013-02-14 10:09:34.640000
2013-02-14 10:11:16.241234   2013-02-14 10:09:34.640000
2013-02-14 10:11:16.497418   2013-02-14 10:09:34.640000
2013-02-14 10:11:16.738275   2013-02-14 10:09:34.640000
2013-02-14 10:11:16.981937   2013-02-14 10:09:34.640000
...
2013-02-14 10:12:53.737133   2013-02-14 10:09:34.640000
2013-02-14 10:12:53.984177   2013-02-14 10:09:34.640000
2013-02-14 10:12:54.262773   2013-02-14 10:09:34.640000
2013-02-14 10:12:54.505545   2013-02-14 10:09:34.640000
2013-02-14 10:12:54.726044   2013-02-14 10:09:34.640000
2013-02-14 10:12:55.010031   2013-02-14 10:09:34.640000
2013-02-14 10:12:55.245294   2013-02-14 10:09:34.640000
2013-02-14 10:12:55.488452   2013-02-14 10:09:34.640000
2013-02-14 10:12:55.737416   2013-02-14 10:09:34.640000
2013-02-14 10:12:55.980422   2013-02-14 10:09:34.640000
2013-02-14 10:12:56.234256   2013-02-14 10:09:34.640000
2013-02-14 10:12:56.471297   2013-02-14 10:09:34.640000
2013-02-14 10:12:56.721042   2013-02-14 10:09:34.640000
2013-02-14 10:12:56.982471   2013-02-14 10:09:34.640000
2013-02-14 10:12:57.218042   2013-02-14 10:09:34.640000
Name: start_time, Length: 416

cues[(cues.component == '/usr/bin/naoqi-bin') & (cues.thread_id == 0)].start_time.dtype
Out[51]: dtype('datetime64[ns]')

cues[(cues.component == '/usr/bin/naoqi-bin') & (cues.thread_id == 0)].start_time.unique()
Out[52]: array([1970-01-16 90:09:34.640000], dtype=datetime64[ns])
```

My version of pandas is 0.10.1.
