df.ix causing exception when index is not from 0
#### Code Sample, a copy-pastable example if possible

When the index is set from 0, df.ix[0,0] works fine.

```
>>> df = pd.DataFrame({'A':[1,2,3], 'B':[10,20,30]},index=[0,1,2])
>>> df
   A   B
0  1  10
1  2  20
2  3  30
>>> df.ix[0,1]
10
```

When the index is set from other 0, df.ix[0,0] cases exception.

```
>>> df2 = pd.DataFrame({'A':[1,2,3], 'B':[10,20,30]},index=[2,3,4])
>>> df2
   A   B
2  1  10
3  2  20
4  3  30
>>> df2.ix[1,0]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "c:\python27\lib\site-packages\pandas\core\indexing.py", line 75, in __ge
titem__
    return self._getitem_tuple(key)
  File "c:\python27\lib\site-packages\pandas\core\indexing.py", line 784, in _ge
titem_tuple
    return self._getitem_lowerdim(tup)
  File "c:\python27\lib\site-packages\pandas\core\indexing.py", line 908, in _ge
titem_lowerdim
    section = self._getitem_axis(key, axis=i)
  File "c:\python27\lib\site-packages\pandas\core\indexing.py", line 1018, in _g
etitem_axis
    return self._get_label(key, axis=axis)
  File "c:\python27\lib\site-packages\pandas\core\indexing.py", line 93, in _get
_label
    return self.obj._xs(label, axis=axis)
  File "c:\python27\lib\site-packages\pandas\core\generic.py", line 1749, in xs
    loc = self.index.get_loc(key)
  File "c:\python27\lib\site-packages\pandas\indexes\base.py", line 1947, in get
_loc
    return self._engine.get_loc(self._maybe_cast_indexer(key))
  File "pandas\index.pyx", line 137, in pandas.index.IndexEngine.get_loc (pandas
\index.c:4066)
  File "pandas\index.pyx", line 159, in pandas.index.IndexEngine.get_loc (pandas
\index.c:3930)
  File "pandas\hashtable.pyx", line 303, in pandas.hashtable.Int64HashTable.get_
item (pandas\hashtable.c:6527)
  File "pandas\hashtable.pyx", line 309, in pandas.hashtable.Int64HashTable.get_
item (pandas\hashtable.c:6465)
KeyError: 1L
```
#### output of `pd.show_versions()`

> > > pd.show_versions()
## INSTALLED VERSIONS

commit: None
python: 2.7.12.final.0
python-bits: 32
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 94 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None

pandas: 0.18.1
nose: None
pip: 8.1.2
setuptools: 20.10.1
Cython: None
numpy: 1.11.1
scipy: None
statsmodels: None
xarray: None
IPython: 5.0.0
sphinx: None
patsy: None
dateutil: 2.5.3
pytz: 2016.4
blosc: None
bottleneck: None
tables: None
numexpr: None
matplotlib: 1.5.1
openpyxl: 2.3.5
xlrd: 1.0.0
xlwt: 1.1.2
xlsxwriter: None
lxml: None
bs4: 4.4.1
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 0.9.8
pymysql: None
psycopg2: None
jinja2: 2.8
boto: None
pandas_datareader: None
