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-extensions/src/main/java/org/jivesoftware/smackx/attention/packet/AttentionExtension.java

@@ -67,7 +67,7 @@ public class AttentionExtension implements ExtensionElement {
      */
     public String toXML() {
         final StringBuilder sb = new StringBuilder();
-        sb.append("<").append(getElementName()).append(" xmlns=\"").append(
+        sb.append('<').append(getElementName()).append(" xmlns=\"").append(
                 getNamespace()).append("\"/>");
         return sb.toString();
     }