The behavior of `DataFrame.ix` slicing with a negative index
#### Code Sample, a copy-pastable example if possible

``` python
>>> df = pd.DataFrame(np.arange(16).reshape(8, 2))
>>> df
    0   1
0   0   1
1   2   3
2   4   5
3   6   7
4   8   9
5  10  11
6  12  13
7  14  15
>>> df[-4:]
    0   1
4   8   9
5  10  11
6  12  13
7  14  15
```

But, `df.ix[-4:]` returns:

``` python
>>> df.ix[-4:]
    0   1
0   0   1
1   2   3
2   4   5
3   6   7
4   8   9
5  10  11
6  12  13
7  14  15
```
#### Expected Output

The following code is the behavior of the python's slicing with negative index.

``` python
>>> x = list(range(10))
>>> x[-5:]
[5, 6, 7, 8, 9]
```

So, I think `df.ix[-4:]` should return the same output as `df[-4:]` like:

``` python
>>> df.ix[-4:]
    0   1
4   8   9
5  10  11
6  12  13
7  14  15
```
#### output of `pd.show_versions()`

``` python
>>> pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 3.5.1.final.0
python-bits: 64
OS: Darwin
OS-release: 15.4.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: ja_JP.UTF-8

pandas: 0.18.1
nose: None
pip: 8.1.2
setuptools: 21.0.0
Cython: None
numpy: 1.11.0
scipy: 0.17.0
statsmodels: None
xarray: None
IPython: 4.2.0
sphinx: None
patsy: None
dateutil: 2.5.3
pytz: 2016.4
blosc: None
bottleneck: None
tables: None
numexpr: None
matplotlib: 1.5.1
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.8
boto: None
pandas_datareader: None
```
