GroupBy Filter does not work for 2+ grouping columns in 0.12.0
Trying to filter out groups with less than 1 row with the new groupby().filter() in 0.12.0:

``` python

df = pd.DataFrame({'A' : ['foo', 'bar', 'foo', 'bar',
   ....:                  'foo', 'bar', 'foo', 'foo'],
   ....:           'B' : ['one', 'one', 'two', 'three',
   ....:                  'two', 'two', 'one', 'three'],
   ....:           'C' : np.random.randn(8),
   ....:           'D' : np.random.randn(8)})

grp = df.groupby(['A', 'B'])

grp.apply(lambda x: len(x)>1) # Returns correct bool series

A    B    
bar  one      False
     three    False
     two      False
foo  one       True
     three    False
     two       True
dtype: bool

grp.filter(lambda x: len(x)>1)

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-140-4dccfb561597> in <module>()
----> 1 grp.filter(lambda x: len(x)>1)

C:\Python27\lib\site-packages\pandas\core\groupby.pyc in filter(self, func, dropna, *args, **kwargs)
   2092                 res = path(group)
   2093 
-> 2094             if res:
   2095                 indexers.append(self.obj.index.get_indexer(group.index))
   2096 

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

```
