ENH: Groupby.plot enhancement
Allow to support `DataFrameGroupBy.plot` and `SeriesGroupBy.plot` covering current `DataFrame.hist(by=XXX)` and `DataFrame.boxplot(by=XXX)` usecases. Implementation is incomplete but covering most of usecases / behaviours.
- [x] fix `FIXME`s
- [x] Split to `plot_seriesgroupby` and `plot_dataramegroupby` specifying default values.
- [x] Refactoring `plot_frame` and `plot_series`, modify `docstring`.  (#8037)
- [x] Make line subplot to accept multiple colors (#9894)
- [x] secondary_y
- [x] error bars (raise `NotImplementedError`)
- [x] split some cleanups required in prior to this PR (#10717)
- [x] doc
- [x] tests
## Following gist contains all patterns of outputs.
- https://gist.github.com/sinhrks/ad805b6689140930d99a
## Rendered doc

![untitled 3](https://cloud.githubusercontent.com/assets/1696302/9026114/83da4e80-395d-11e5-86f0-caef498d16d8.jpg)
