sort_index(axis=1) doesn't sort integers
Original:

```python
In [45]: a
Out[45]:
            commits
                 -1       -3       -2
auth_date
2013-01-04  42217.0  19804.0  37007.0
2013-01-05  44793.0  37007.0  42217.0

In [46]: a.sort_index(axis=1)
Out[46]:
            commits
                 -1       -3       -2
auth_date
2013-01-04  42217.0  19804.0  37007.0
2013-01-05  44793.0  37007.0  42217.0

In [47]: a.sort_index(axis=1, level=1)
Out[47]:
            commits
                 -1       -3       -2
auth_date
2013-01-04  42217.0  19804.0  37007.0
2013-01-05  44793.0  37007.0  42217.0

In [48]: a.sort_index(axis=1, ascending=False)
Out[48]:
            commits
                 -2       -3       -1
auth_date
2013-01-04  37007.0  19804.0  42217.0
2013-01-05  42217.0  37007.0  44793.0
```
However I couldn't clearly recreate with a copy-pastable version:
```python
In [49]: print a.to_dict()
{('commits', -3): {Timestamp('2013-01-05 00:00:00', freq='D'): 37007.0, Timestamp('2013-01-04 00:00:00', freq='D'): 19804.0}, ('commits', -1): {Timestamp('2013-01-05 00:00:00', freq='D'): 44793.0, Timestamp('2013-01-04 00:00:00', freq='D'): 42217.0}, ('commits', -2): {Timestamp('2013-01-05 00:00:00', freq='D'): 42217.0, Timestamp('2013-01-04 00:00:00', freq='D'): 37007.0}}

In [50]: from pandas import Timestamp, DataFrame

In [51]: b = DataFrame({('commits', -3): {Timestamp('2013-01-05 00:00:00', freq='D'): 37007.0, Timestamp('2013-01-04 00:00:00', freq='D'): 19804.0}, ('commits', -1): {Timestamp('2013-01-05 00:00:00', fr
    ...: eq='D'): 44793.0, Timestamp('2013-01-04 00:00:00', freq='D'): 42217.0}, ('commits', -2): {Timestamp('2013-01-05 00:00:00', freq='D'): 42217.0, Timestamp('2013-01-04 00:00:00', freq='D'): 37007.
    ...: 0}})

In [52]: b
Out[52]:
            commits
                 -3       -2       -1
2013-01-04  19804.0  37007.0  42217.0
2013-01-05  37007.0  42217.0  44793.0

In [54]: b.sort_index(axis=1, ascending=False)
Out[54]:
            commits
                 -1       -2       -3
2013-01-04  42217.0  37007.0  19804.0
2013-01-05  44793.0  42217.0  37007.0
```
#### Problem description

Well, I expected sorting of the MultiIndex columns.

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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.13.final.0
python-bits: 64
OS: Darwin
OS-release: 16.3.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: en_US.UTF-8
LANG: en_US.UTF-8
LOCALE: None.None

pandas: 0.19.2
nose: 1.3.7
pip: 9.0.1
setuptools: 28.6.1
Cython: 0.25.2
numpy: 1.11.3
scipy: 0.18.1
statsmodels: 0.6.1
xarray: 0.9.1
IPython: 5.1.0
sphinx: 1.5.1
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2016.10
blosc: None
bottleneck: 1.2.0
tables: 3.3.0
numexpr: 2.6.1
matplotlib: 2.0.0
openpyxl: 2.4.1
xlrd: 1.0.0
xlwt: 1.2.0
xlsxwriter: 0.9.6
lxml: 3.7.2
bs4: 4.5.3
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.1.5
pymysql: None
psycopg2: 2.6.2 (dt dec pq3 ext lo64)
jinja2: 2.9.4
boto: 2.45.0
pandas_datareader: None

</details>
