BUG: LWOM-SUN (LastWeekOfMonth) should directly work / doc examples of use
http://pandas.pydata.org/pandas-docs/stable/timeseries.html#dateoffset-objects
is their

Isn't 'LWOM-SUN' supposed to work for the offset? 

it raises `ValueError`

```
In [5]: date_range('20130101',periods=5,freq=pd.offsets.LastWeekOfMonth(weekday=pd.offsets.WeekDay.SUN))
Out[5]: 
<class 'pandas.tseries.index.DatetimeIndex'>
[2013-01-27, ..., 2013-05-26]
Length: 5, Freq: LWOM-SUN, Timezone: None

In [6]: date_range('20130101',periods=5,freq=pd.offsets.LastWeekOfMonth(weekday=pd.offsets.WeekDay.SUN)).tolist()
Out[6]: 
[Timestamp('2013-01-27 00:00:00', offset='LWOM-SUN'),
 Timestamp('2013-02-24 00:00:00', offset='LWOM-SUN'),
 Timestamp('2013-03-31 00:00:00', offset='LWOM-SUN'),
 Timestamp('2013-04-28 00:00:00', offset='LWOM-SUN'),
 Timestamp('2013-05-26 00:00:00', offset='LWOM-SUN')]
```
