BUG: grouped hist raises AttributeError with single group
Includes 3 minor fixes for `hist`.
- `DataFrame.hist` raises `AttributeError` when the target column specifiedwith `by` kw only contains a single value.

```
import pandas as pd
import numpy as np
import matplotlib.pyplot as plt
import pandas.util.testing as tm

n=100
gender = tm.choice(['Male', 'Female'], size=n)
classroom = tm.choice(['A', 'B', 'C'], size=n)
single = tm.choice(['S'], size=n)

df = pd.DataFrame({'gender': gender,
                   'classroom': classroom,
                   'single': single, 
                   'height': np.random.normal(66, 4, size=n),
                   'weight': np.random.normal(161, 32, size=n),
                   'category': np.random.randint(4, size=n)})

df.hist(by='single')
# AttributeError: 'AxesSubplot' object has no attribute 'ravel'
```
- `hist` can accept `rot` kw only when `by` is specified, but `rot` actually does nothing. I understand `rot` value should be used for `xticklabels` rotation if `xrot` is not specified?

```
df.hist(rot=45)
# AttributeError: Unknown property rot
# -> This is OK because we can use xrot kw

df.hist(by='classroom', rot=45)
# It raises no error, but ticks are NOT rotated.
```
- `hist` always displays `xticklabels` on all axes even if specifying `sharex=True`. Other plots only draws `xticklabels` on bottom axes if `sharex` is True.

@TomAugspurger ?
 For me, `df.hist(by='classroom', rot=45)` _does_ rotate the xlabels.

I'd say `rot` should be a synonym for `xrot in hist. If you one of them, use that for the`xticklabels`. If you see both raise a`ValueError`. Make sure it's consistent for`hist_frame`,`hist_series`, and`grouped_hist`.
 Strange. The latest master seems to output following result, and both `grouped_hist` and `_grouped_plot` doesn't have `rot` logic. I'm not sure on what part xticks can be rotated in current code.

```
axes = df.hist(by='classroom', rot=30)
```

![figure_1](https://cloud.githubusercontent.com/assets/1696302/3137995/521a2034-e867-11e3-9a6c-8d1aec257f9e.png)

Whatever, how about remove `rot` fix from this PR? I only cared to get `grouped_hist` worked assuming it is a special case of `grouped_hist`. To make plot functions consistent, it is better to use same logic as the`plot` and `boxplot` (not only `hist`). I'll open separate issue once organized.
 @sinhrks pls rebase
 this looks ok, @TomAugspurger ?
