ENH: add rounding method to DatetimeIndex/TimedeltaIndex
xref #8184

to say round by minute
http://stackoverflow.com/questions/17781159/round-pandas-datetime-index

by hour
https://github.com/pydata/pandas/issues/4314

might be simpler to use numpy astype here
http://stackoverflow.com/questions/23315163/merging-two-pandas-timeseries-shifted-by-1-second

```
In [75]: index = pd.DatetimeIndex([ Timestamp('20120827 12:05:00.002'), Timestamp('20130101 12:05:01'), Timestamp('20130712 15:10:00'), Timestamp('20130712 15:10:00.000004') ])

In [79]: index.values
Out[79]: 
array(['2012-08-27T08:05:00.002000000-0400',
       '2013-01-01T07:05:01.000000000-0500',
       '2013-07-12T11:10:00.000000000-0400',
       '2013-07-12T11:10:00.000004000-0400'], dtype='datetime64[ns]')

In [78]: pd.DatetimeIndex(((index.asi8/(1e9*60)).round()*1e9*60).astype(np.int64)).values
Out[78]: 
array(['2012-08-27T08:05:00.000000000-0400',
       '2013-01-01T07:05:00.000000000-0500',
       '2013-07-12T11:10:00.000000000-0400',
       '2013-07-12T11:10:00.000000000-0400'], dtype='datetime64[ns]')
```
