More flexibility in to_period
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```
Also noticed that `to_period` doesn't work for frequency QS:

In [110]: q = pd.date_range('01-Jan-2012', periods=8, freq='QS-Jan')

In [111]: p = q.to_period()
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call
last)
<ipython-input-111-869731392d28> in <module>()
----> 1 p = q.to_period()

C:\dev\bin\Python27\lib\site-packages\pandas\tseries\index.pyc in
to_period(self, freq)
    582             freq = self.freqstr
    583
--> 584         return PeriodIndex(self.values, freq=freq)
    585
    586     def order(self, return_indexer=False, ascending=True):

C:\dev\bin\Python27\lib\site-packages\pandas\tseries\period.pyc in
__new__(cls, data, ordinal, freq, start, end, periods, copy, name,
year, month, quarter, day, hour, minute, second)
    510                 hour=None, minute=None, second=None):
    511
--> 512         freq = _freq_mod.get_standard_freq(freq)
    513
    514         if periods is not None:

C:\dev\bin\Python27\lib\site-packages\pandas\tseries\frequencies.pyc
in get_standard_freq(freq)
    501         return get_offset_name(freq)
    502
--> 503     code, stride = get_freq_code(freq)
    504     return _get_freq_str(code, stride)
    505

C:\dev\bin\Python27\lib\site-packages\pandas\tseries\frequencies.pyc
in get_freq_code(freqstr)
     64
     65     base, stride = _base_and_stride(freqstr)
---> 66     code = _period_str_to_code(base)
     67
     68     return code, stride

C:\dev\bin\Python27\lib\site-packages\pandas\tseries\frequencies.pyc
in _period_str_to_code(freqstr)
    687         return _period_code_map[freqstr]
    688     except:
--> 689         alias = _period_alias_dict[freqstr]
    690         return _period_code_map[alias]
    691

KeyError: 'QS-JAN'

In [112]: pd.__version__
Out[112]: '0.8.1'

Although this can be worked around by providing freq='Q' as an
argument I think it would be sensible if the `to_period` method would
automatically recognise QS as a quarterly frequency.

Regards,
Dave
```
