Added 'File' to get 'Cleaner' testable
Activated Travis PHP 5.4 and 5.5 again
Use 'phpbu\Backup\File' instead of 'SplFileInfo'
Accept 0 as valid size
Inclrease testability by adding file system functions to 'Target'
Added 'fileExists' and 'unlink' methods to 'Target'
Added 'isWritable' method to 'File' class
Make use of the new 'Target' methods 'fileExists' and 'unlink'
Added 'deleteTarget' option to delete all backups
Even the currenty created backup is deleted,
this may only makes sense in combination with a configured 'Sync'
Just edited some whitespaces