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

[Droppers]

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.