Addition of two DateOffsets
It is convenient to be able to add one DateOffset to another one:

``` python
from pandas.tseries.offsets import DateOffset as DO
DO(hour=2) + DO(hour=3) # raises ValueError
```

Currently it is not supported. Note that `DateOffset`s basically replicates the functionality of `relativedelta`s from `dateutil` module which support addition

``` python
from dateutil.relativedelta import relativedelta as RD
RD(hour=2) + RD(hour=3) # yields relativedelta(hour=3)
```
