add trunked upload implemented .

add handy function to use trunkedUpload api :
func (api *DropboxApi) UploadByChunked(localPath, path string, trunkSize, retryCount int) (*PathMetadata, *ApiError)
func (api *DropboxApi) UploadReaderByChunked(file io.Reader, path string, trunkSize, retryCount int) (*PathMetadata, *ApiError)