BUG: query with local strings gives KeyError
From SO question (http://stackoverflow.com/questions/23974664/unable-to-query-a-local-variable-in-pandas-0-14-0), referencing a local variable gives an error when this is a string (workaround seems to be to wrap it in a list):

```
In [39]: fills = pd.DataFrame({'Symbol':['BUD US', 'BUD US', 'IBM US', 'IBM US'], 'Price':[109.70, 109.72, 183.30, 183.35]})

In [40]: fills
Out[40]: 
    Price  Symbol
0  109.70  BUD US
1  109.72  BUD US
2  183.30  IBM US
3  183.35  IBM US

In [41]: my_symbol = ['BUD US']

In [42]: fills.query('Symbol==@my_symbol')
Out[42]: 
    Price  Symbol
0  109.70  BUD US
1  109.72  BUD US

In [43]: my_symbol = 'BUD US'

In [44]: fills.query('Symbol==@my_symbol')
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-44-c16846018a13> in <module>()
----> 1 fills.query('Symbol==@my_symbol')

...

/home/joris/scipy/pandas/pandas/computation/expr.pyc in _rewrite_membership_op(self, node, left, right)
    341             # of one string, kind of a hack
    342             if right_str:
--> 343                 self.env.remove_tmp(right.name)
    344                 name = self.env.add_tmp([right.value])
    345                 right = self.term_type(name, self.env)

/home/joris/scipy/pandas/pandas/computation/scope.pyc in remove_tmp(self, name)
    287             The name of a temporary to be removed
    288         """
--> 289         del self.temps[name]
    290 
    291     @property

KeyError: '__pd_eval_local_my_symbol'
> /home/joris/scipy/pandas/pandas/computation/scope.py(289)remove_tmp()
    288         """
--> 289         del self.temps[name]
    290 
```

@cpcloud 
