Feature request: Add annual offset that is not anchored
#### Code Sample, a copy-pastable example if possible

```python
pd.bdate_range(end='today', periods=2, freq='1A').freq.name
# Prints 'A-DEC'
pd.bdate_range(end='today', periods=2, freq='1A')[0].date()
# prints datetime.date(2015, 12, 31)
```
#### Problem description

`Pandas` divides offsets into `regular` and `anchored` (see [Offset aliases](http://pandas.pydata.org/pandas-docs/stable/timeseries.html#offset-aliases)).

There are `regular` aliases for any frequency but annual. A typical use case - give me a `bdate_range` with dates spaced 1 year apart ending today. `12M` and `52W` are only proxies, but won't give the exact expected result.

#### Expected Output
```python
pd.bdate_range(end='2017-03-12', periods=2, freq='1A-actual-for-real')[0].date() # Saturday
# prints datetime.date(2016, 3, 11) # Friday

```


#### Output of ``pd.show_versions()``

<details>
pandas: 0.19.2
</details>
