BUG: Fix bad axis limits when boxplotting with 'by' (#7528)
closes #5517

The original bug involved the y-limits for boxplots being wrong when using `'by': turns out it has to do with the dummy axis that gets created when the plots don't fill the axis layout, e.g. when we're plotting 3 columns in a 2 by 2 layout.

The fix for this basically involves not setting the `sharey`/`sharex` argument for blank/dummy axes.

I've tried to test this in a couple of different ways, but some feedback on better ways to test would be good. In particular, I'm not sure if it's okay to check the `_sharey` attribute of the axes, given that it's private.

Example of a plot that previously failed:

``` python
import numpy as np
import pandas as pd
import matplotlib.pyplot as plt

np.random.seed(100)

n_rows = 200
example_df = pd.DataFrame({
    'count1': np.random.exponential(50, n_rows).astype(int),
    'count2': np.random.exponential(100, n_rows).astype(int),
    'count3': np.random.exponential(20, n_rows).astype(int),
    'season': np.random.randint(1, 5, n_rows)
})
example_df['totalcount'] = example_df['count1'] + example_df['count2']

# Bad limits - grid not filled out
example_df.boxplot(['count1', 'count2', 'totalcount'], by='season')
plt.show()
```

![boxplot_axes](https://cloud.githubusercontent.com/assets/1460294/4068707/b14abeaa-2e3b-11e4-94ce-8ebb226d7adb.png)
 What do you mean when you say it previously failed? This is what I get on master:

![master](https://cloud.githubusercontent.com/assets/1312546/4070242/d2843fee-2e57-11e4-9f67-baf018105408.png)
 Oh wow, sorry, it took me a while to get around to this, I had the fix sitting there but had to do up the test cases. It's quite possible it was fixed somewhere else in the meantime but previously it was acting like was described in #7528 (specifically the second image there). If master is now giving the correct axis limits, maybe #7528 can just be closed.

Could also keep the test cases here in case it ever pops up again?
 Current master still outputs problematic result using matplotlib 1.3.1. Thus it is nice to have a fix and explicit test for this.

This should close #5517 also. 
 @onesandzeroes pls add a release not that closing #5517 as well. rebase / squash and good 2 go.
 @jreback Added the release note, rebased and squashed. Should be all good.
 @TomAugspurger merge when u r ok with this
 Great, thanks!
