Autobackup improvements

Added some improvements for handling of the AutoBackup folder. (Sorry for putting it all into one pull request and changing a lot of white spaces. I'm new to git.)
- Increased max photo count which can be downloaded from an album to 10000.
- Added photo id to the file name to avoid duplicates (I had a lot in the Autobackup folder)
- Do not try to delete special folders when --purge is enabled
- Fixed path splitting issues on Windows