Bug-22: Add Word and WordStorage class

@john-hu Please help me review this patch. Thank you.
Should we always send `DOWNLOAD_COMPLETE` to UI, even if an error occurred? If we don't want to see it, we should move this line to line 53 after `copyFileFromUrl()` function call. With that, the UI may know if any error occurred. And the meaning of DOWNLOAD_COMPLETE is the download procedure is done correctly.