index name in DataFrame.to_latex() ends up in the columns section
#### Code Sample, a copy-pastable example if possible

```
In [2]: df = pd.DataFrame([[1,2], [3,4]], index=['a', 'b'], columns=['A', 'B'])

In [3]: df.index.name = 'indexname'

In [4]: print(df.to_latex())
\begin{tabular}{lrr}
\toprule
{} &  A &  B \\
indexname &    &    \\
\midrule
a         &  1 &  2 \\
b         &  3 &  4 \\
\bottomrule
\end{tabular}
```
#### Problem description

Maybe it's just a matter of taste (or maybe there are conventions which I missed), but I think it would be nicer if the ``indexname &    &    \\`` line was _below_ the  ``\midrule`` line, or alternatively, if an additional ``\midrule`` was placed above it. This is particularly annoying when columns are a ``MultiIndex`` with level names, because the index name really seems the name of a(n empty) level rather than of the index.

```
In [2]: df = pd.DataFrame([[1,2], [3,4]], index=pd.Index(['a', 'b'], name='myindex'),
   ...:                   columns=pd.MultiIndex.from_tuples([(0,1), (0,2)], names=['first', 'second']))

In [3]: print(df.to_latex())
\begin{tabular}{lrr}
\toprule
first & \multicolumn{2}{l}{0} \\
second &  1 &  2 \\
myindex &    &    \\
\midrule
a       &  1 &  2 \\
b       &  3 &  4 \\
\bottomrule
\end{tabular}
```

#### Expected Output

```
In [3]: print(df.to_latex())
\begin{tabular}{lrr}
\toprule
first & \multicolumn{2}{l}{0} \\
second &  1 &  2 \\
\midrule
myindex &    &    \\
a       &  1 &  2 \\
b       &  3 &  4 \\
\bottomrule
\end{tabular}
```
or
```
In [3]: print(df.to_latex())
\begin{tabular}{lrr}
\toprule
first & \multicolumn{2}{l}{0} \\
second &  1 &  2 \\
\midrule
myindex &    &    \\
\midrule
a       &  1 &  2 \\
b       &  3 &  4 \\
\bottomrule
\end{tabular}
```


#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.5.3.final.0
python-bits: 64
OS: Linux
OS-release: 4.7.0-1-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: it_IT.utf8
LOCALE: it_IT.UTF-8

pandas: 0.19.0+783.gcd35d22a0
pytest: 3.0.6
pip: 9.0.1
setuptools: 33.1.1
Cython: 0.25.2
numpy: 1.12.0
scipy: 0.18.1
xarray: 0.9.1
IPython: 5.1.0.dev
sphinx: 1.4.9
patsy: 0.3.0-dev
dateutil: 2.5.3
pytz: 2016.7
blosc: None
bottleneck: 1.2.0
tables: 3.3.0
numexpr: 2.6.1
feather: 0.3.1
matplotlib: 2.0.0
openpyxl: 2.3.0
xlrd: 1.0.0
xlwt: 1.1.2
xlsxwriter: 0.9.6
lxml: 3.7.1
bs4: 4.5.3
html5lib: 0.999999999
sqlalchemy: 1.0.15
pymysql: None
psycopg2: None
jinja2: 2.8
s3fs: None
pandas_gbq: None
pandas_datareader: 0.2.1


</details>
