Offset bug for n != 1?
From stackoverflow

https://stackoverflow.com/questions/22835671/pandas-resample-with-seasonal-frequency#

Should this be True?

```
 [~/]
 [18]: from pandas.tseries.offsets import QuarterBegin

 [~/]
 [19]: ts = pd.Timestamp('2014-1-1')

 [~/]
 [20]: offset = QuarterBegin(2, startingMonth=4)

 [~/]
 [21]: offset.onOffset(ts)
 [21]: True
```

`onOffset` doesn't do anything with `n`, so it think January is on the offset when only April and October are expected to be on the offset. Shouldn't onOffset know about n? I can look into this, if it should. I'd like to refamilarize myself with this code a bit.
