DateRange for 12 month not created
Target: create a 12 month date range:

base data

```
rng = DateRange('1/1/2000 00:00', periods=8760, offset=datetools.Hour())
ts = Series(randn(len(rng)), index=rng)
```

the range

```
ts.index[0]
Out[95]: datetime.datetime(2000, 1, 1, 0, 0)

ts.index[-1]
Out[96]: datetime.datetime(2000, 12, 30, 23, 0)

mrg = DateRange(start=ts.index[0], end=ts.index[-1], offset=datetools.MonthEnd())

mrg
Out[98]: 
<class 'pandas.core.daterange.DateRange'>
offset: <1 MonthEnd>, tzinfo: None
[2000-01-31 00:00:00, ..., 2000-11-30 00:00:00]
length: 11
```

Why do I not get 12 months? Jan-Dec?
And how could I get a range out of 12 months from the ts?
