Wrong string comparison in df.eval
Why string comparison return 'False'? How I can compare strings?

```
In [3]: import pandas as pd

In [4]: df = pd.DataFrame( {'col1':[1,0]} )

In [5]: external_var = 'yes'

In [6]: df.eval( '(@external_var == "yes") == col1' )
Out[6]:
0    False
1     True
dtype: bool

In [7]: df.eval( '(@external_var == "yes")')
Out[7]: False

In [8]: df.eval( '(1 == 1)')
Out[8]: True

In [9]: df.eval( '("yes" == "yes")')
Out[9]: False

In [11]: import numexpr as ne

In [12]: pd.__name__, pd.__version__, ne.__name__, ne.__version__
Out[12]: ('pandas', '0.16.2', 'numexpr', '2.4.3')
```
