More dynamic plugin directory lookup

I installed Qvitter in local/plugins/ rather than plugins/ which
meant that some statically assigned paths were incorrect. This patch
will instead find the current Qvitter directory when the plugin is
loaded and use that when finding ctime for files on the filesystem.

I'd probably recommend using some kind of incremental VERSION tag instead, but I guess with rolling updates in a git repo, that's a lot of unnecessary commits for version bumping ;)