Implement functions for `with` statement and `close()`

I noticed that the `ZipFile` will stay open (OS prevents it from being modified or removed) and it is currently not possible to use it in a `with` statement. So I'd suggest implementing `__enter__` , `__exit__` and a `close()` function.

At the moment you will have to call `apk.zip.close()` manually, to figure this out you need to look at the code.