Rule: Warn on use of Math and JSON as functions

This rule should warn when it encounters the use of object properties of the global object (`Math` and `JSON`) as functions:

    var x = Math();
    var y = JSON();

See http://jslinterrors.com/a-is-not-a-function/

---

I'm unsure of the rule name... I've called it "no-obj-calls" for now but I'm open to suggestions.