add tests for query/eval with groupby to officially support it
right now it "works", but it's not tested:

```
In [34]: df = DataFrame({'gender': tm.choice(['Male', 'Female'], size=n), 'height': random.normal(66, 4, size=n)})
In [35]: df
Out[35]:
   gender  height
0  Female  75.295
1    Male  64.490
2    Male  68.218
3    Male  62.713
4    Male  63.967
5  Female  69.106
6    Male  61.441
7    Male  69.658
8    Male  67.151
9  Female  64.251

In [36]: df.groupby('gender').query('61 < height < 65')
Out[36]:
          gender  height
gender
Female 9  Female  64.251
Male   1    Male  64.490
       3    Male  62.713
       4    Male  63.967
       6    Male  61.441
```
