Add checkstyle rule for StringBuilder.append(char)

And replace all instances where String.Builder.append() is called with a
String of length one with append(char).

Also adds StringUtils.toStringBuilder(Collection, String).

smack-im/src/main/java/org/jivesoftware/smack/roster/RosterEntry.java

@@ -207,7 +207,7 @@ public final class RosterEntry extends Manager {
                 group = iter.next();
                 buf.append(group.getName());
             }
-            buf.append("]");
+            buf.append(']');
         }
         return buf.toString();
     }