Added NoAvatar option/class, which is now the default

src/AvatarFetcher.java

@@ -16,8 +16,7 @@ public AvatarFetcher(CodeSwarmConfig cfg) {
   }
 
   public String fetchUserImage(String username) {
-    //Override fetchUserImage in your Avatar Fetcher
-    return null;
+    throw new RuntimeException("Override fetchUserImage in your Avatar Fetcher");
   }
 
   protected static String getFilename(String key){

src/NoAvatar.java

@@ -0,0 +1,11 @@
+
+public class NoAvatar extends AvatarFetcher {
+
+  public NoAvatar(CodeSwarmConfig cfg) {
+    super(cfg);
+  }
+
+  public String fetchUserImage(String username) {
+    return null;
+  }
+}

src/code_swarm.java

@@ -243,7 +243,7 @@ public void setup() {
 
     isInputSorted = cfg.getBooleanProperty(CodeSwarmConfig.IS_INPUT_SORTED_KEY, false);
 
-    avatarFetcher = getAvatarFetcher(cfg.getStringProperty("AvatarFetcher","AvatarFetcher"));
+    avatarFetcher = getAvatarFetcher(cfg.getStringProperty("AvatarFetcher","NoAvatar"));
 
     /**
      * This section loads config files and calls the setup method for all physics engines.