Added handling of DropBox move API command for remote file moving

Hi all,

for a backup process I set up I have added the move function to your shell script. You can merge it into your main branch if you find it useful,

kind regards
chof

PS.: Functionality was added to access the Dropbox API move function:

- new API Url to access move (API_MOVE_URL)
- new function db_move executing a remove move
- new command rename|move taking the soure/old file and the target/new
file as parameters