Bug indexing a DataFrame with MultiIndex through .ix
Hi,

here is an example that demonstrates the error I'm getting (using pandas 0.9.2.dev-61766ec):

Example script:

``` python
from pandas import DataFrame, MultiIndex

index_columns = list("abc")
df = DataFrame([[0, 1, 0, "x"], [0, 0, 1, "y"]],
               columns=index_columns + ["data"])
df = df.set_index(index_columns)
print "Test DataFrame:"
print df
print

query_index = df.index[:1]  # first index
print "MultiIndex to index the DataFrame through .ix[...]:"
print repr(query_index)
print

print "This works:"
print df.ix[query_index]["data"]
print

print "This doesn't:"
print df.ix[query_index, "data"]
print
```

which outputs:

```
Test DataFrame:
      data
a b c     
0 1 0    x
  0 1    y

MultiIndex to index the DataFrame through .ix[...]:
MultiIndex
[(0, 1, 0)]

This works:
a  b  c
0  1  0    x
Name: data

This doesn't:
Traceback (most recent call last):
  File "pandas_bug_github.py", line 20, in <module>
    print df.ix[query_index, "data"]
  File "/usr/local/lib/python2.7/dist-packages/pandas-0.9.2.dev_61766ec-py2.7-linux-x86_64.egg/pandas/core/indexing.py", line 32, in __getitem__
    return self._getitem_tuple(key)
  File "/usr/local/lib/python2.7/dist-packages/pandas-0.9.2.dev_61766ec-py2.7-linux-x86_64.egg/pandas/core/indexing.py", line 202, in _getitem_tuple
    return self._getitem_lowerdim(tup)
  File "/usr/local/lib/python2.7/dist-packages/pandas-0.9.2.dev_61766ec-py2.7-linux-x86_64.egg/pandas/core/indexing.py", line 287, in _getitem_lowerdim
    loc = ax0.get_loc(tup[0])
  File "/usr/local/lib/python2.7/dist-packages/pandas-0.9.2.dev_61766ec-py2.7-linux-x86_64.egg/pandas/core/index.py", line 2080, in get_loc
    return self._get_level_indexer(key, level=0)
  File "/usr/local/lib/python2.7/dist-packages/pandas-0.9.2.dev_61766ec-py2.7-linux-x86_64.egg/pandas/core/index.py", line 2182, in _get_level_indexer
    loc = level_index.get_loc(key)
  File "/usr/local/lib/python2.7/dist-packages/pandas-0.9.2.dev_61766ec-py2.7-linux-x86_64.egg/pandas/core/index.py", line 674, in get_loc
    return self._engine.get_loc(key)
  File "engines.pyx", line 113, in pandas.lib.IndexEngine.get_loc (pandas/src/tseries.c:124894)
  File "engines.pyx", line 115, in pandas.lib.IndexEngine.get_loc (pandas/src/tseries.c:124554)
TypeError
```

The funny thing is that if, for instance, we swap the rows in the dataframe ( [[0, 0, 1, "y"], [0, 1, 0, "x"]] instead of [[0, 1, 0, "x"], [0, 0, 1, "y"]] ) it works just fine.

Regards.
