Use mod times, since we have them now, to bypass md5 check if we can.
Can invoke backups retain by itself, or with upsync.
Upsync doesnt upload files it will delete on backups retain. Updated README.

Removed option to do backups_retain alone since such an occurence is so rare.
Uncomment delete. Add dry run option.
Added dry run option.
Fixed bug where no remote file crashed metadata check.