A some default excludes

In some cases there can be leftover backup files from other plugins. We should not back those up, plus it can cause issues if total size is too important.

- [x]  [UpdraftPlus](http://wordpress.org/plugins/updraftplus/)
- [ ]  [BackUpBuddy](http://ithemes.com/purchase/backupbuddy/)
- [ ]  [BackWPUp](http://wordpress.org/plugins/backwpup/)
- [x]  [Online Backup for WordPress](http://wordpress.org/plugins/wponlinebackup/)
- [x]  [Duplicator](http://wordpress.org/plugins/duplicator/)
- [ ]  [Xcloner](http://wordpress.org/plugins/xcloner-backup-and-restore/)
- [x]  WPDBBackup (stored in `WP_BACKUP_DIR` constant)
- [x] `.svn/`
- [x] `.git/`

**Todos**

- [x] Check that the folders exist before excluding them
- [x] Don't allow the default exclude rules to be removed