df.query() doesn't follow python truthy-ness?
``` python
In [44]: df=pd.DataFrame([[0,10],[1,20]],columns=['cat','count'])
    ...: df
Out[44]: 
   cat  count
0    0     10
1    1     20

In [45]: df.query('cat')
Out[45]: 
   cat  count
0    0     10
1    1     20
```

Expected the first row where cat==0 to be dropped since 0 is Falsey.

This unhelpful exception is how I stumbled over this

```
In [46]: df.query('cat & count > 10')
NotImplementedError: couldn't find matching opcode for 'and_blb'
```
