Add example for background operation and synchronous transaction

[Zhuinden]

The example managed to end up using only realm.executeTransactionAsync().

On background threads, you're supposed to use the following pattern:

public Void doInBackground(Void... nothing) {
    // start of background thread
    Realm realm = null;
    try {
         realm = Realm.getDefaultInstance();
         realm.executeTransaction(new Realm.Transaction() {
              @Override
              public void execute(Realm realm) {
                  // do execute
              }
         });
    } finally {
        if(realm != null) {
             realm.close();
        }
    }
    // end of background thread
}

This example is missing from the current sample code.

[Zhuinden]

Background operations are shown in https://github.com/Zhuinden/xkcd-example/ (DownloadTask)