BUG: subtraction of datetime / timedelta should be allowed
```
In [12]: s2
Out[12]:
0   1 days
1   2 days
2   3 days
dtype: timedelta64[ns]

In [9]: pd.Timestamp('2012-01-01') + s2
Out[9]:
0   2012-01-02
1   2012-01-03
2   2012-01-04
dtype: datetime64[ns]

In [10]: pd.Timestamp('2012-01-01') - s2
TypeError: can only operate on a timedelta/DateOffset and a datetime for addition, but the operator [__rsub__] was passed

In [11]: pd.Timestamp('2012-01-01') + (-s2)
Out[11]:
0   2011-12-31
1   2011-12-30
2   2011-12-29
dtype: datetime64[ns]
```
