bool DataFrame arith operations error when size is large enough
``` python
import pandas as pd
import numpy as np

N = 1000000
bools = np.random.randn(N, 1) > 0

df = pd.DataFrame(bools)

df * df
```

```
/python2.7/site-packages/numexpr/necompiler.pyc in typeCompileAst(ast)
    147                 raise NotImplementedError(
    148                     "couldn't find matching opcode for '%s'"
--> 149                     % (ast.value + '_' + retsig+basesig))
    150         # First just cast constants, then cast variables if necessary:
    151         for i, (have, want)  in enumerate(zip(basesig, sig)):

NotImplementedError: couldn't find matching opcode for 'mul_bbb'
```

I'm guessing there's a size trigger that throws ops to `numexpr` since this doesn't fail for smaller frames. 
