don't disconnect from tag on reset or poweroff

fixes #21

remote-reader/app/src/main/java/com/vsmartcard/remotesmartcardreader/app/screaders/NFCReader.java

@@ -49,23 +49,31 @@ public class NFCReader implements SCReader {
     private static int TIMEOUT = 10000;
     @Override
     public void powerOn() throws IOException {
+        if (!card.isConnected()) {
             card.connect();
             card.setTimeout(TIMEOUT);
         }
-
-    @Override
-    public void powerOff() throws IOException {
-        card.close();
     }
 
     private static byte[] SELECT_MF = {(byte) 0x00, (byte) 0xa4, (byte) 0x00, (byte) 0x0C};
-    @Override
-    public void reset() throws IOException {
+    private void selectMFifConnected() throws IOException {
+        if (card != null && card.isConnected()) {
             byte[] response = card.transceive(SELECT_MF);
             if (response.length == 2 && response[0] == 0x90 && response[1] == 0x00) {
                 Log.d(this.getClass().getName(), "Resetting the card by selecting the MF results in " + Hex.getHexString(response));
             }
         }
+    }
+
+    @Override
+    public void powerOff() throws IOException {
+        selectMFifConnected();
+    }
+
+    @Override
+    public void reset() throws IOException {
+        selectMFifConnected();
+    }
 
     /* calculation based on https://code.google.com/p/ifdnfc/source/browse/src/atr.c */
     @Override