fix some little bugs

1.ACWebDAVDownloadRequest.h&.m
  add a new var to hold the reference of currentConnection, used to
cancel a request if needed;
  also add a new method(cancel) to cancel this request

2.ACWebDAVDeleteRequest.m
   - connection: didReceiveResponse: response.statusCode=201
      OR response.statusCode=204 all shows deleted successfully.

3.ACWebDAVClient.h  - add new optional delegate methods
- (BOOL)needToCancelDownload;
- (void)resetCancelDownload:(BOOL)cancel;
- (BOOL)needToCancelUpload;
- (void)resetCancelUpload:(BOOL)cancel;
used to check if need to cancel a request