BUG: hidden ticklabels with sharex and secondary
Closes https://github.com/pydata/pandas/issues/9158

``` python
import pandas as pd
d = {'A' : [1., 2., 3., 4.], 'B' : [4., 3., 2., 1.], 'C': [5, 1, 3, 4]}
df = pd.DataFrame(d, index=pd.date_range('2014 10 11', '2014 10 14'))

axes = df[['A','B']].plot(subplots=True)
df['C'].plot(ax=axes[0], secondary_y=True)   # x tick labels dissappear when this line is executed
```

![gh](https://cloud.githubusercontent.com/assets/1312546/7216419/ffe3bc2e-e5c4-11e4-89de-d18e5e59d3b0.png)

Introduced in the subplot refactoring in https://github.com/pydata/pandas/pull/7457

cc @sinhrks

The fix is pretty simple, but I'm worried I may have broken other things. No tests failed though, so hopefully we're good.

My reformatting hides the change.

The only difference is in the call to `_handle_shared_axes`, `naxes` is now `nrows * ncols`, instead of `len(all_axes)`. The problem was that `all_axes` is 3 now, since there's the original two subplots and the additional 3rd plot, which is getting plotted on the first subplot. This confused `_handle_shared_axes`.
 looks good. minor doc issue.
 @TomAugspurger looks ok, pls rebase.
 needs a rebase
 Rebased... will merged later today.
 The failure [here](https://travis-ci.org/pydata/pandas/jobs/59048983) was a build failure on a job that doesn't run the plotting code. Going to merge
 Well, I messed that up. Pushed before `--continuing` my rebase, and it looks like github has it's done with this PR
 this problem seems to be still existing in pandas 0.17.0, on subplots with at least two rows:

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

mat = np.random.rand(10,2)
df = pd.DataFrame(mat, columns=['A','B'])

fig, axes = plt.subplots(nrows=2, ncols=2)                                 
df.plot(secondary_y='B', ax=axes[0, 1], legend=False)
```

![image](https://cloud.githubusercontent.com/assets/5053963/10861467/dd738f4e-7f56-11e5-8db9-e30582765b8f.png)
