Remove the dependency on javax.xml.transform

Not all platforms support this API (e.g. Android). Strip the
prettyPrint() method since it wasn't used anyway.

Also move XmlUtil in a more appropriate package.

core/src/main/java/org/jivesoftware/smack/util/XmlUtils.java

@@ -0,0 +1,34 @@
+/**
+ *
+ * Copyright the original author or authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.jivesoftware.smack.util;
+
+/**
+ * Simple utility for XML.
+ * 
+ * @author Robin Collier
+ */
+public class XmlUtils {
+
+    static public void appendAttribute(StringBuilder builder, String att, String value) {
+        builder.append(" ");
+        builder.append(att);
+        builder.append("='");
+        builder.append(value);
+        builder.append("'");
+    }
+
+}