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)

@TomAugspurger have a look at this as well
 Agreed that this should be the default.

@dstephens99 Could you rebase? And while you're rebasing, could you change the docstring for fontsize to make it clear that the parameter affects x and y ticklabels?
 @TomAugspurger  Done.  Also specified that rotation only happens to the x ticks on a vertical plot and the y ticks on a horizontal plot.
 @dstephens99 can you rebase

@TomAugspurger @jorisvandenbossche ?
 Apparently someone else's pull request was merged to fix this (#8940).
 ahh I c; I think was done at the london hackathon ....

next time :)
 @dstephens99 Ah, sorry, didn't see your PR. 
However, if you still want to rebase it, that would be valuable. The actual fix is the same, but you also fixed some docstrings and tests that were not in the other PR that still can go in (only the whatsnew entry you will have to remove, the rest should be ok after a rebase).
 @dstephens99 can ypu rebase / update according to @jorisvandenbossche comment. thanks
 @jreback Green and good to go on this one.
 @dstephens99 Thanks!
