.groupby by should indicate it aligns the passed in Series
#### Code Sample, a copy-pastable example if possible

```python
>>> series = pd.Series([1, 2, 3], index=range(3))

>>> key = pd.Series([0, 0, 1], index=range(3, 6))  # Note: disjunct index

>>> series.groupby(key).sum()
Series([], dtype: int64)

>>> series.groupby(key.values).sum()
0    3
1    3
dtype: int64
```
#### Problem description

The current behavior is a bug because the [.groupby() documentation](https://github.com/pandas-dev/pandas/blob/v0.19.2/pandas/core/generic.py#L3948-L3949) says about `by` parameter:
> If a dict or Series is passed, the Series or dict **VALUES** will be used to determine the groups

Above example shows this not being the case.

#### Expected Output

With above setup:
```py
>>> series.groupby(key).sum()
0    3
1    3
dtype: int64
```

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

<details>
pandas: 0.19.0+416.ge1390cd
</details>
