String comparison in query()
Hi, it seems that string comparisons aren't supported in query() yet, so maybe this isn't a bug yet. Anyway, hopefully this behavior will be fixed for future editions of pandas.

``` python
import pandas as pd
import numexpr as ne

a = list('abcdef')
b = range(6)
df = pd.DataFrame({'X':pd.Series(a),'Y': pd.Series(b)})

df_Y = df.query('Y < 3')             ## Works fine.
ne_works = ne.evaluate('"a" < "d"')  ## ne_works == np.array([True])
df_X = df.query('X < "d"')           ## RuntimeError: max recursion depth exceeded
```
