[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.*

Now we are saving the data in the cache. So it will persist on the device even after the request is complete and the Blob is garbage collected. This is a change in behavior.

You could instead create a temp file (`[TiUtils createTempFile:ext]`) and return the temp file as part of the event dict as well. That way once the event is delivered and processed the file will automatically be deleted once it goes out of scope.