BUG: fontsize parameter of plot only affects one axis.

fixes #8765 

```python
import pandas as pd
df = pd.DataFrame(np.random.randn(10, 9), index=range(10))
df.plot(fontsize=7)
```
![fig_fonts_all_same_size](https://cloud.githubusercontent.com/assets/5957850/4969745/86e42902-686b-11e4-959c-e22b544e63e2.png)