DatetimeIndex gives strange answer when trying to find intervals
The following is strange to me:

```
>>> import datetime
>>> import pandas
>>> dates = [datetime.datetime(2012, 5, 1), datetime.datetime(2012, 5, 2), datetime.datetime(2012, 5, 3)]
>>> index = pandas.DatetimeIndex(dates)
>>> index[1:] - index[:-1]
    <class 'pandas.tseries.index.DatetimeIndex'>
    [2012-05-03 00:00:00]
    Length: 1, Freq: None, Timezone: None
```

I would expect something that would represents the 2 intervals.  I thought at first a PeriodIndex, but that will not work in this case.  Probably though should return a DateOffset?  Maybe?

Note if using the datetime representation from above:

```
>>> import numpy as np
>>> npdates = np.array(dates)
>>> npdates[1:] - npdates[:-1]
    array([1 day, 0:00:00, 1 day, 0:00:00], dtype=object)
```

Kindest regards,
Tim
