BUG: timeseries subplots may display unnecessary minor ticklabels
Related to #7457. The fix was incomplete because it only hides major ticklabels, not minor ticklabels. This causes incorrect result in time-series plot.

(And another problem is `rot` default is not applied to minor ticks. I'll check this separatelly)

```
df = pd.DataFrame(np.random.randn(10, 4), index=pd.date_range(start='2014-07-01', freq='M', periods=10))
df.plot(subplots=True)
```
### Current result:

Minor ticklabels of top 3 axes are not hidden.
![figure_ng](https://cloud.githubusercontent.com/assets/1696302/3634314/36ebc68c-0f22-11e4-9f9c-da803dd27d5e.png)
### Result after fix:

![figure_ok](https://cloud.githubusercontent.com/assets/1696302/3634865/57a28f1a-0f4e-11e4-847f-fff225740439.png)
