BUG: impossible to select string with special character from DataFrame via query
Field (columns) not accept special character to filter via query, sample:

Source:

```
df = DataFrame({'Licença': [1, 2, 3, 4]})
df.query("Licença == 1")
```

Error:

```
Traceback (most recent call last):
  File "/home/avelino/.virtualenvs/mining/lib/python2.7/site-packages/tornado/web.py", line 1192, in _stack_context_handle_exception
    raise_exc_info((type, value, traceback))
  File "/home/avelino/.virtualenvs/mining/lib/python2.7/site-packages/tornado/web.py", line 1375, in wrapper
    result = method(self, *args, **kwargs)
  File "/home/avelino/.virtualenvs/mining/lib/python2.7/site-packages/tornado/gen.py", line 146, in wrapper
    result = func(*args, **kwargs)
  File "/home/avelino/projects/mining/mining/views.py", line 83, in open
    df = df.query(df_generate(df, self.get_argument(f), f))
  File "/home/avelino/.virtualenvs/mining/lib/python2.7/site-packages/pandas/core/frame.py", line 1803, in query
    res = self.eval(expr, **kwargs)
  File "/home/avelino/.virtualenvs/mining/lib/python2.7/site-packages/pandas/core/frame.py", line 1854, in eval
    return _eval(expr, **kwargs)
  File "/home/avelino/.virtualenvs/mining/lib/python2.7/site-packages/pandas/computation/eval.py", line 207, in eval
    truediv=truediv)
  File "/home/avelino/.virtualenvs/mining/lib/python2.7/site-packages/pandas/computation/expr.py", line 762, in __init__
    self.terms = self.parse()
  File "/home/avelino/.virtualenvs/mining/lib/python2.7/site-packages/pandas/computation/expr.py", line 781, in parse
    return self._visitor.visit(self.expr)
  File "/home/avelino/.virtualenvs/mining/lib/python2.7/site-packages/pandas/computation/expr.py", line 442, in visit
    node = ast.fix_missing_locations(ast.parse(clean))
  File "/opt/python2.7/lib/python2.7/ast.py", line 37, in parse
    return compile(source, filename, mode, PyCF_ONLY_AST)
  File "<unknown>", line 1
    Licença == 'Ativo por kit'
```
