Provide a smoother upgrade path for existing LottaNZB installations

The 0.3 release of LottaNZB forced those people who use the source package, to decide whether there was already an existing LottaNZB installation or not and choose the appropriate setup.py command. This led to some confusion (https://answers.launchpad.net/lottanzb/+faq/139).

The problem is that "./setup.py install" doesn't overwrite any files by default. So if there's an existing LottaNZB installation, the "install" command won't work at all. Having a separate "upgrade" method used to remove an existing installation is not a good idea (usability-wise).

Proposal: The "install" method looks for an existing installation and asks the user whether to upgrade it or not. We also need to provide a flag used to skip this detection when building RPM, Gentoo or Debian packages etc., which could be merged with the skip-post-installation flag (?).