Code generation produces invalid JavaScript

The default code generation for an AST that calls a function node is in broken.

In the following example...

```javascript
var src = "(function() { return 23; })()";
var src2 = lively.ast.Parser.parse(src, "topLevel").asJS();  //  "function() { return 23 }()";
```

...`src2` omits the parantheses around the function literal which produces code that cannot be parsed. This effects both Chrome and Firefox.

The attached patch fixes this bug.