MultiIndex with Heterogenous dtype
I'm not sure if this is supposed to work or not.

I'm reading in a csv file:

`df = pd.read_csv('nc201052.dat', index_col=['FLOW', 'PERIOD', 'DECLARANT', 'PARTNER', 'PRODUCT_NC'], nrows=1000)`

```
df.head(5)

                                          STAT_REGIME  VALUE_1000ECU  QUANTITY_TON  SUP_QUANTITY
FLOW PERIOD DECLARANT PARTNER PRODUCT_NC                                                        
1    201052 001       3       01                    4       41818.21       13419.4             0
                      4       01                    4       17667.97        3939.6             0
                      5       01                    4        6956.63        1181.9             0
                      6       01                    4       44011.98        1031.2             0
                      7       01                    4        7141.68         559.0             0

```

The 'DECLARANT'  columns index type is 'object' (they coded one item as 'EU'). 

`df.index[0]` gives `(1, 201052, '001', 3, '01')`, but `df.ix[1, 201052, '001']` raises [this](https://gist.github.com/b735fad192ce878dc353) error.

When I only import the first 10 rows, the parser infers the "DECLARANT" column as an integer, and the slice works.  That's what is leading me to guess that it's the type of the index that's messing things up.  

Sorry if this isn't actually a bug.  Still very new to python.  Thanks!
