Selection on MultiIndex wrong after applying .iloc[::n] on unsorted Series/DataFrame 
Hi,
When slicing a Series/DataFrame which has a multilevel index which is not sorted on the first level, selecting on the first level with .ix or .loc returns an empty dataframe instead of the true values.

``` python
In [524]: s = pd.Series(range(30),index=pd.MultiIndex.from_arrays([["A"]*10+["C"]*10+["B"]*10,range(30)]))
In [526]: x = s.iloc[::3]
In [527]: x
Out[527]:
A  0      0
   3      3
   6      6
   9      9
C  12    12
   15    15
   18    18
B  21    21
   24    24
   27    27
dtype: int64
In [528]: x.ix["B"]
Out[528]: Series([], dtype: int64)
```

It works if "C" and "B" are exchanged in In [524].
## INSTALLED VERSIONS

commit: None
python: 2.7.3.final.0
python-bits: 64
OS: Linux
OS-release: 3.2.0-70-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.14.0
nose: 1.3.3
Cython: 0.20.2
numpy: 1.8.1
scipy: 0.14.0
statsmodels: None
IPython: 2.1.0
sphinx: None
patsy: None
scikits.timeseries: None
dateutil: 2.2
pytz: 2014.4
bottleneck: None
tables: 3.1.1
numexpr: 2.4
matplotlib: 1.3.1
openpyxl: 1.8.6
xlrd: None
xlwt: 0.7.5
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
bq: None
apiclient: None
rpy2: None
sqlalchemy: 0.9.8
pymysql: 0.6.2.None
psycopg2: None
