Some descriptive comments (not JavaDoc) to task classes

common/src/main/java/io/bitsquare/common/Clock.java

@@ -7,6 +7,8 @@ import java.util.LinkedList;
 import java.util.List;
 import java.util.concurrent.TimeUnit;
 
+// Helps configure listener objects that are run by the `UserThread` each second
+// and can do per second, per minute and delayed second actions.
 public class Clock {
     private static final Logger log = LoggerFactory.getLogger(Clock.class);
 

common/src/main/java/io/bitsquare/common/MasterTimer.java

@@ -7,6 +7,7 @@ import java.util.Set;
 import java.util.TimerTask;
 import java.util.concurrent.CopyOnWriteArraySet;
 
+// Runs all listener objects periodically in a short interval.
 public class MasterTimer {
     private final static Logger log = LoggerFactory.getLogger(MasterTimer.class);
     private static final java.util.Timer timer = new java.util.Timer();

common/src/main/java/io/bitsquare/common/UserThread.java

@@ -27,6 +27,7 @@ import java.util.Random;
 import java.util.concurrent.Executor;
 import java.util.concurrent.TimeUnit;
 
+// Helps run delayed and periodic actions in the caller thread.
 public class UserThread {
     private static final Logger log = LoggerFactory.getLogger(UserThread.class);
     private static Class<? extends Timer> timerClass;