Adding in DeleteArgs, PrepareDelete, and Delete

This adds in a collection of missing delete connivance functions.
In Client/kv:
- kv.Delete
- kv.PrepareDelete

In proto/api
DeleteArgs

These should compliment the current put functions in those files.
Actually, you should remove this part of the PR. I think it's a bit too much to have individual calls like this in the client. We only do it for PutProto and PreparePutProto because of the inconvenience of having to marshal the proto data.