resample time series with 2 quarters
see this question on so: http://stackoverflow.com/questions/14569223/timegrouper-pandas

``` python
In [213]: idx = pd.date_range('2008-07-01', '2010-12-01', freq='MS')

In [214]: s = pd.Series(np.random.randn(idx.size), index=idx)

In [215]: s.index[0] + pd.datetools.QuarterEnd(startingMonth=12) * 2
Out[215]: <Timestamp: 2008-12-31 00:00:00>

In [216]: s.resample(pd.datetools.QuarterEnd(startingMonth=12) * 2)
Out[216]: 
2008-09-30   -0.220156
2009-03-31   -0.392138
2009-09-30   -0.720243
2010-03-31    0.261541
2010-09-30    0.444490
2011-03-31   -0.093383
Freq: 2Q-DEC
```

So adding the offset to the starting point of the time series leads to the time desired, while resampling starts one quarter before.
