Aggregation with 'any' always returns True
When running the code below I’d expect the value for a=1 to be False:

```
In [33]: DataFrame({'a': [0, 1], 'b': [True, False]}).groupby('a').agg(any)
Out[33]: 
      b
a      
0  True
1  True
```

pandas.**version** == '0.7.2'
