Don't overwrite existing files on the uploading target

[Ghostofapacket]

Added --ignore-existing to prevent overwrite issues as per issue #305

[JustAnotherArchivist]

This needs more care: if a file already exists on the remote server, the local file is simply deleted since rsync's exit status does not indicate the skip in any way. I'm not sure what the correct/best action in that case is, but that's most definitely not it... Detecting the situation is also not easy. In a few quick tests, it didn't seem possible to do so easily. Apparently increasing the verbosity and parsing the output would be required for that.