Preserve timestamps and permissions

Add a `--preserve` option to keep both timestamps and permissions. This closes #31, except for the owner part. I don't think that preserving the owner is really important as it is only possible while being root and this should not be really a use case for unoconv.

I have only tested with Python3 has I don't have UNO binding for Python 2 anymore but the code should be compatible with Python 2. Please test if you can.