(WIP) BUG/CLN: Better timeseries plotting / refactoring tsplot
Must be revisited after #7717.
#6608 seems to be solved by following 4 fixes.
- [ ] `PeriodIndex` should support the same freqs as `DatetimeIndex` (Related to #7222, maybe solved by #5148)
- [ ] Better logic to find common divisor frequency (try to use `tsplot` as much)
- [x] When plotting with `x_compat` to the `ax` which already holds `tsplot` lines, `tsplot` lines must be redrawn on `x_compat` coordinates.
  - [ ] Check whether `to_timestamp(how='e')` always revert `PeriodIndex` back to original `DatetimeIndex`
- [x] If target ax already holds `x_compat` lines, continuous plot should be drawn on `x_compat` coordinates (current version already have a logic, but cannot work if `ax` once have `freq` property).

Other refactoring:
- [x] Do not re-plot every row in `tsplot`
- [x] Simplify `LinePlot` flow (separated as #7717)
- [x] Store plot_func for line and area mixed time-series plot  (separated as #7733)

Result using current PR (modified #6608 a little to show legend).

```
s1 = pd.Series([1, 2, 3], index=[datetime.datetime(1995, 12, 31),
                                 datetime.datetime(2000, 12, 31),
                                 datetime.datetime(2005, 12, 31)], name='idx1')
s2 = pd.Series([1, 2, 3], index=[datetime.datetime(1997, 12, 31),
                                 datetime.datetime(2003, 12, 31),
                                 datetime.datetime(2008, 12, 31)], name='idx2')

ax = s1.plot(legend=True)
ax = s2.plot(legend=True)
s1.plot(ax=ax, legend=True)
```

![figure_1](https://cloud.githubusercontent.com/assets/1696302/3486598/477df0e8-0445-11e4-873b-dc619adaa92a.png)

One question is whether `tsplot` is categorized as public function? If so, I'll prepare separate func.
