Dropbox batch delete support

Currently deletions on Dropbox make one request per file, which can result in constantly hitting rate limits when deleting a lot of files. Batch is already supported for uploads, I would suggest extending it to deletion as well. The relevant API call is documented at HTTP - Developers - Dropbox.

Can you make an issue about this please? We'd have to generalise the batcher a bit but I think it could be done.

Maybe you'd like to work on it?