Naive fixes for issues #6, #7

Issue #6 - I implemented `del` by just setting the given key to nil. This wouldn't work as might be expected for slices.

Issue #7 - It seems that the panic was due to `isinstance()` attempting to check the function object's `__class__` attribute, but Lua doesn't allow indexing a function. So I added a type check. I think `isinstance()` still works correctly. I call the fix naive because maybe you can mess around with metatables (from Lua code) and make indexing work for functions anyway, and then my type check would prevent `LuaObject_getattr` from working.