del should call __del__ method

Example test with custom object:

``` Python
class Foo():
    def __del__(self):
        print('__del__')

f = Foo()
del f  # should print '__del__'
```

Also for builtin types with finaliser, like socket.socket.
Example case:

``` Python
s = socket.socket()
s.connect(addr)  # binds "hard" socket to object `s`
...
del s  # s.close() has not been called - "hard" socket will never be released
```
