Safety measure for getBackups()

[sergejostir]

This fixes pretty severe bug, which happens, if we use a folder which is not empty for backupPath.

Current implementation simply returns all content of the folder, making no check, which content is actually relevant.

Example:

You set backupPath to '/' and then use deleteBackups(). Since getBackups will return all files, the package will then remove your whole project ... which we don't want :)

My fix makes sure that only files that are actual backup files created by this package are taken into consideration.