add support for file_bytes argument with managed_file_context()

[bosd]

This is an attempt to cherry-pick in the PR from #15.

Was really fighting with git and the merge commits on the fork of a fork and so on...
Cherry pick approach was the cleanest.

I hope to have solved the merge conflicts correctly.
Please review..

[foarsitter]

Some of my thoughts:

1. Adding the extra variable file_bytes feels for me more like an extra type for filepath.
2. In __init__ there are two places where we raise an InvalidArguments
3. filepath isn't used beyond open(self.filepath, "rb"), so why do we need a name attribute for file_bytes?
4. The naming of managed_file_context isn't directly clear to me, open() would, or even implement PDFHandler as contextmanager so we can do with self but I think with self.open() would be better
5. Is @contextmanager doing the cleanup magically or do we need to do that manually?