BUG: sqrt not implemented in df.eval 
In the following scenario:

```
import pandas as pd
import numpy as np

a = np.random.rand(1000)
df = pd.DataFrame({'a' : a })
```

This call doesn't work (while it is supposed to, I guess)

```
df.eval('sqrt(a)')

     NotImplementedError: 'Call' nodes are not implemented
```

while this one does

```
df.eval('(a)**(.5)')
```

I guess this is a bug.
