Zip files

Closes #43 

This merge will add the ability to toggle zipped archives on/off in the addon settings. This will alter the regular backup archive creation as follows: Backup archive will be created locally as a zip file in the special://temp directory. This will require space on the local drive. Zipped archive will then be uploaded to the remote destination with a .zip extension. 

This is in contrast the normal backup flow where the archive files are written directly to the remote destination. For restores the reverse process will be used. The only major piece that cannot be avoided is that xbmcbackup.val files cannot be checked within archives when they are being listed in the selection window. This is because the zip archive cannot be accessed reliably once it has been moved to the remote source. The existence of this file will be checked for before a restore operation proceeds. This will only really affect the backup rotation schemes since anything in the remote location that matches the naming scheme with a zip extension is assumed to be an archive.