Series.resample('1S', how='last') on series with dtype=datetime64[ns] is very slow
example test:

``` python
intSeries = pd.Series(5, pd.date_range(start='2000-01-01', end='2000-01-05', freq='555000U'), dtype='int64')
timeSeries = intSeries.astype('datetime64[ns]')

time0 = time.time()
intSeries.resample('1S', how='last')
print 'resampling the int series took ', str(timedelta(seconds = time.time()-time0)), '(H:M:S)'

time0 = time.time()
timeSeries.resample('1S', how='last')
print 'resampling the datetime64[ns] series took ', str(timedelta(seconds = time.time()-time0)), '(H:M:S)'
```

output shows 14sec vs 0.5sec:
resampling the int series took  **0:00:00.054000** (H:M:S)
resampling the datetime64[ns] series took  **0:00:14.350000** (H:M:S)
