DataFrame.plot() with secondary_y is missing y-axis gridlines, has x-axis gridlines above data
Info:
matplotlib (1.4.3)
pandas (0.16.2)
Python 3.4.2

Example:

```
dataframe.plot(secondary_y=['error'])
```

Yields:
![image](https://cloud.githubusercontent.com/assets/395851/9832544/249e58bc-594c-11e5-83d8-457d0080b1c1.png)

The gridlines appear on top of the `power` data and there are no lines parallel to the y-axis.

Expected:
![image](https://cloud.githubusercontent.com/assets/395851/9832567/c749c358-594c-11e5-83c5-2c54daa75ca8.png)

Which was obtained by the following code:

```
ax = dataframe.plot(y=['power'])
ax2 = dataframe.plot(y=['error'], secondary_y=True, mark_right=False, ax=ax)
ax.grid(True)
ax2.grid(False)
ax2.yaxis.set_major_locator(LinearLocator(len(ax.get_yticks())))
ax2.yaxis.set_major_formatter(FuncFormatter(lambda x, pos: '{:0.2f}'.format(x)))
```
