[TIMOB-17502] iOS: URLSession DownloadCompleted event now returns file url instead of TiBlob

JIRA: https://jira.appcelerator.org/browse/TIMOB-17502

**To test:**
```
// Require in the urlSession module
var urlSession = require("com.appcelerator.urlSession");
var session;
 
// App UI
var win = Ti.UI.createWindow({backgroundColor:"white"});
var progress = Ti.UI.createProgressBar({
    width: 200,
    height: 50,
    min: 0,
    max: 1,
    value: 0,
    style: Ti.UI.iPhone.ProgressBarStyle.PLAIN,
    top: 10,
    message: 'Downloading image URL',
    font: { fontSize: 12, fontWeight: 'bold'},
    color: '#888'
});
win.add(progress);
var imageView = Ti.UI.createImageView({
        top:150,
          height:300,
          width:200
    });
win.add(imageView);
 
var button = Ti.UI.createButton({
    title:'Download Image (url)',
    height:40,
    width:200,
    top:70
});
 
button.addEventListener('click', function()
{
	// Check if the device is running iOS 8 or later, before registering for local notifications
	if (Ti.Platform.name == "iPhone OS" && parseInt(Ti.Platform.version.split(".")[0]) >= 8) {
    	Ti.App.iOS.registerUserNotificationSettings({
	    	types: [
            	Ti.App.iOS.USER_NOTIFICATION_TYPE_ALERT,
            	Ti.App.iOS.USER_NOTIFICATION_TYPE_SOUND,
            	Ti.App.iOS.USER_NOTIFICATION_TYPE_BADGE
        	]
    	});
	}
    // Create a session configuration
    // The string parameter is an arbitrary string used to identify the session in events
    var sessionConfig = urlSession.createURLSessionBackgroundConfiguration("com.appcelerator.test");
    // Create a session
    session = urlSession.createURLSession(sessionConfig);
    // Create a background download task to get the asset with the URL
    //urlSession.backgroundDownloadTaskWithURL(session,"https://raw.github.com/appcelerator-developer-relations/KitchenSink/master/Resources/images/dog@2x~iphone.jpg"); //small file
 	//urlSession.backgroundDownloadTaskWithURL(session,"http://imgsrc.hubblesite.org/hu/db/images/hs-2004-07-a-full_jpg.jpg"); //medium sized file
 	urlSession.backgroundDownloadTaskWithURL(session,"http://download.thinkbroadband.com/1GB.zip"); //huge file
    progress.show();
});
 
win.add(button);
win.open();
 
// Monitor this event to receive updates on the progress of the download
Ti.App.iOS.addEventListener("downloadprogress", function(e) {
    // Update the progress indicator
    progress.value = (e.totalBytesWritten/e.totalBytesExpectedToWrite);
});

// Monitor this event to know when the download completes
Ti.App.iOS.addEventListener("downloadcompleted", function(e) {
    Ti.API.info("download completed " + JSON.stringify(e));
 
    // Copy file from cache
 	var cacheFile = Ti.Filesystem.getFile(e.url);
	var downloadedFile = Ti.Filesystem.getFile(Ti.Filesystem.applicationDataDirectory, cacheFile.getName());
	downloadedFile.write(cacheFile.read()); // copy and save from cache
	Ti.API.info("downloadedFile: " + downloadedFile.getNativePath());
	// set new Image
	// imageView.setImage(downloadedFile.read());
    // Invalidate the session and cancel current session tasks
    urlSession.invalidateAndCancel(session);
 
    // Notify the user the download is complete if the application is in the background
	var notification = Ti.App.iOS.scheduleLocalNotification({
    	alertBody: "Download completed!",
	    badge: 1,
 		sound: "/alert.wav",
    	date: new Date(new Date().getTime())
	});     
    progress.hide();
});

// Monitor this event to know when all session tasks have completed
Ti.App.iOS.addEventListener('sessioncompleted', function(e) {
    Ti.API.info("sessioncompleted " + JSON.stringify(e));
    if (e.success) {
        alert("Downloads completed successfully.");
    }
});
```
- Create a new Project
- Double-click the Project's "tiapp.xml" file to open it in the Overview tab.
- In the Modules section, click the Add button (green plus sign) to open the Mobile Modules dialog.
- Locate and select "com.appcelerator.urlSession". 
- Click OK.
- Copy the code segment above to the project "app.js" file
- Run the Project 
- *Note: the link downloaded is 1GB in size in the test code above. Replace with commented out lines of code linked to smaller files if necessary.*

If copying fails I would return a blob with data in memory