Added push for existing archive

Hi,

please take a look at the added functionality. It allows to upload existing archive of DB onto S3. We much prefer to have local backups given that it will take us much time to pull data out of S3. So uploading of existing archive  eliminates the need to backup twice - once for local copy and one more time for S3.

Do you think it may be added to the mainstream branch (we are open for refactoring because still working on it)?

Thanks for you work anyway!