Crosstab margins not calculating correctly
The margins of the crosstab are not calculated correctly when the row data is based on a filtered dataframe command.  Below is an example.  You will notice that the table_filtered does not calculate column totals correctly.  I am using pandas 0.13.1, python 3.3.2, Windows 7.

``` python
# Create dataframe
d = {'Fruit' : pd.Series(['Apples', 'Apples', 'Apples', 'Apples', 'Oranges', 'Oranges', 'Oranges', 'Oranges']), \
'Method' : pd.Series(['Abstract', 'Surreal', 'Abstract', 'Surreal', 'Abstract', 'Surreal', 'Abstract', 'Surreal'])}

df=pd.DataFrame(d)


# Create full table with correct margin calculation
table_full=pd.crosstab( \
    [df.Fruit], \
    [df.Method], \
    margins=True) 

# Create table based on filtered database.  Notice margins are no longer correct.
table_filtered=pd.crosstab( \
    [df.Fruit[df.Fruit=='Apples']], \
    [df.Method], \
    margins=True) 
```

Output:
![crosstab](https://cloud.githubusercontent.com/assets/7172629/2618912/14f99bfa-bc28-11e3-94a9-ab998242f0d2.png)
