Changes do Notification and WebImageDownloader

NOTIFICATION
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-Fixed a lot of problems in the NotificationHandler class and GCMlib.
-Included HOWTO use NotificationHandler for Android

WebImageDownloader
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-Image download can now be done in two ways:
- downloadImage() <- puts task on a queue, callback can be assigned.
Images are downloaded sequentially. (Good for mobile)
- downloadImageCoroutine() <- yields until image is downloaded. task is
NOT queued. Images are downloaded assynchronously.