BUG/CLN: LinePlot uses incorrect xlim when secondary_y=True

`xlim` is not set properly when `secondary_y=True`. This is different issue from #7322.

Also, refactored `secondary_y` to be handled only by `MPLPlot`, because it will be required in future fix to pass multiple axes to `df.plot`, like #7069 (I'm willing to work soon..).

```
df = pd.DataFrame(np.random.randn(5, 5), columns=['A', 'B', 'C', 'D', 'E'])
df.plot(secondary_y=True)
```
![figure_1](https://cloud.githubusercontent.com/assets/1696302/3278423/15d4ba78-f3be-11e3-9d8e-172c9d0b754b.png)

```
df = pd.DataFrame(np.random.randn(5, 5), columns=['A', 'B', 'C', 'D', 'E'])
df.plot(secondary_y=['A', 'B'], subplots=True)
```
![figure_2](https://cloud.githubusercontent.com/assets/1696302/3278424/1b2dae44-f3be-11e3-8b8c-b19f5ae7fef3.png)