Error when parsing ExtSlice #17

When parsing complex slicing expressions, like classes[:, :-1], I get this error:
File "/usr/local/lib/python3.4/dist-packages/mutpy/codegen.py", line 498, in visit_ExtSlice for idx, item in node.dims: TypeError: 'Slice' object is not iterable

I solved it replacing the visit_ExtSlice method with this:
` def visit_ExtSlice(self, node):

    for i in range(0, len(node.dims)):
        if i > 0:
            self.write(', ')
        self.visit(node.dims[i])`