ENH: Use self values in Series groupby
#### Is your feature request related to a problem?

I wish I could use pandas to do

```
s = pd.Series([1,2,1,3,1,2])
s.groupby()
```
rather than

```
s.groupby(s)
```

#### Describe the solution you'd like

I would like pandas to interpret the series values as `grouper` when `by` or `level` is not provided.

#### API breaking implications

I can't think of any reason this will affect the API.

#### Describe alternatives you've considered

Grouping using series itself as `grouper`.

`s.groupby(s)`
