Moved version checking to background

Hi there, I had some trouble with the way your library was blocking to download the version file. I've modified it to use an asynctask to check the version and to extend observable to notify the application when this action is finished.
Because this breaks compatibility with API 2, I've bumped version in my code to 2.1

There's a quick integration howto here:
https://github.com/Kiirani/AppaholicsUpdateChecker/wiki/API-2.1-Integration-Guide