Allow multiple backups of the same app

[mvglasow]

Currently oandbackup allows only one backup per app; any existing backups will be overwritten by a new one (see also #261).

As a developer who occasionally tries out different features but needs to go back to the last stable version, I would find it really helpful to be able to keep multiple backups around.

Folder names could be distinguished by appending a timestamp. Choosing a separator character between package name and timestamp that is illegal in a package name would avoid name collisions—or create one subdirectory per backup underneath the package directory.

As a bonus, allow for a short description for each backup (could be stored in a text file in the backup dir).