BUG:  fontsize parameter of plot only affects x-axis.
``` python
import pandas as pd
import numpy as np
df = pd.DataFrame(np.random.randn(10, 9), index=range(10))
df.plot()
```

Results in this plot (all the same font size):
![figure_with_normal_font](https://cloud.githubusercontent.com/assets/5957850/4969656/a577b85c-6866-11e4-8f5a-9861650cb7ae.png)

``` python
df.plot(fontsize=4)
```

Results in this plot (only the x-axis font size is changed):
![figure_with_small_font](https://cloud.githubusercontent.com/assets/5957850/4969658/c1d949de-6866-11e4-91cb-4495242e45b9.png)
