BUG: Addition raises TypeError if Period is on rhs
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

`Period` addition raises `TypeError` if `Period` is on right hand side.

```
1 + pd.Period('2011-01', freq='M')
# TypeError: unsupported operand type(s) for +: 'int' and 'pandas._period.Period'
```
#### Expected

```
1 + pd.Period('2011-01', freq='M')
# Period('2011-02', 'M')
```
