QS frequency alias
The table at http://pandas.pydata.org/pandas-docs/dev/timeseries.html#offset-aliases
appears to suggest there is a QS frequency
but when I try it I get the following traceback:

```
In [84]: pd.date_range('01-Jan-2012', periods=8, freq='QS')
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call
last)
<ipython-input-84-26002af2c5f5> in <module>()
----> 1 pd.date_range('01-Jan-2012', periods=8, freq='QS')

C:\dev\bin\Python27\lib\site-packages\pandas\tseries\index.pyc in
date_range(start, end, periods, freq, tz, normalize, name)
   1315     """
   1316     return DatetimeIndex(start=start, end=end,
periods=periods,
-> 1317                          freq=freq, tz=tz,
normalize=normalize, name=name)
   1318
   1319

C:\dev\bin\Python27\lib\site-packages\pandas\tseries\index.pyc in
__new__(cls, data, freq, start, end, periods, copy, name, tz,
verify_integrity, normalize, **kwds)
    148         if not isinstance(freq, DateOffset):
    149             if freq != 'infer':
--> 150                 freq = to_offset(freq)
    151             else:
    152                 freq_infer = True

C:\dev\bin\Python27\lib\site-packages\pandas\tseries\frequencies.pyc
in to_offset(freqstr)
    390                     delta = delta + offset
    391         except Exception:
--> 392             raise ValueError("Could not evaluate %s" %
freqstr)
    393
    394     if delta is None:

ValueError: Could not evaluate QS

In [85]: pd.date_range('01-Jan-2012', periods=8, freq='QS-JAN')
Out[85]:
<class 'pandas.tseries.index.DatetimeIndex'>
[2012-01-01 00:00:00, ..., 2013-10-01 00:00:00]
Length: 8, Freq: QS-JAN, Timezone: None

In [86]:


QS-JAN seems to work but I'd expect QS to be an alias for QS-JAN in
the same way Q is an alias for Q-DEC:


In [87]: pd.date_range('01-Jan-2012', periods=8, freq='Q-DEC')
Out[87]:
<class 'pandas.tseries.index.DatetimeIndex'>
[2012-03-31 00:00:00, ..., 2013-12-31 00:00:00]
Length: 8, Freq: Q-DEC, Timezone: None

In [88]: pd.date_range('01-Jan-2012', periods=8, freq='Q')
Out[88]:
<class 'pandas.tseries.index.DatetimeIndex'>
[2012-03-31 00:00:00, ..., 2013-12-31 00:00:00]
Length: 8, Freq: Q-DEC, Timezone: None
```
