Expressions don't evaluate when using named parameters
It seems that the eval function doesn't work when using named parameters:

```python
    >>> import pandas.core.computation.eval as eval
    >>> def func(a=None): return a*a
    ... 
    >>> eval.eval('func(a=2)')
    /expr.py", line 314, in visit
    return visitor(node, **kwargs)
    File "python3.6/site-packages/pandas/core/computation/expr.py", line 581, in visit_Call_35
    kwargs.append(ast.keyword(
    AttributeError: 'dict' object has no attribute 'append'
```
Looks like some neglected code in `expr.py`. 