MultiIndex fails when passed a numpy.int64 index element instead of an int
MultiIndex does not behave as expected anymore on 0.8.0 (I'm on Python 3.2.3). In particular, I get a strange error when I pass in an np.int64 object (i.e. the same type as the first level of the index). I hope the following testcase is clear, tried to stick close to the MultiIndex example in the docs.

Code: 

```
from pandas import Series
import numpy as np
arrays = [[0, 0, 1, 1, 2, 2, 3, 3],
          [0, 1, 0, 1, 0, 1, 0, 1]]
s = Series(np.random.randn(8), index=arrays)
print(s[0]) # Works as expected
print(s[np.int64(0)]) # Fails strangely
```

Error:

```
In [6]: print(s[0]) # Works as expected
0   -0.420831
1   -1.818968

In [7]: print(s[np.int64(0)]) # Fails strangely
-1.81896761059---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-7-29c87a86dbe4> in <module>()
----> 1 print(s[np.int64(0)]) # Fails strangely

ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()
```
