fixed a bug in TOTP calculation.

see https://keepass2android.codeplex.com/workitem/279

(if Math.pow(10,6) returns 999 999.99999 this was converted to 999 999 in the previous version)

src/keepass2android/Totp/Totp_Client.cs

@@ -114,7 +114,7 @@ namespace OtpProviderClient
                 | ((hash[offset + 2] & 0xff) << 8)              //Math.
                 | (hash[offset + 3] & 0xff);                    //Math.
 
-            int password = binary % (int)Math.Pow(10, _Length); //Math.
+            int password = binary % Convert.ToInt32(Math.Pow(10, _Length)); //Math.
             return password.ToString(new string('0', _Length)); //Math.
         }
     }