Rework Smack Provider design

this is the first stop towards fixing "SMACK-65: parsing should look for
depth", by providing the initial parsing depth to the provider. Some
methods (.e.g parseMessage) now use the depth as abort condition,
instead of a unclean String equals check.

parseIQ() and parseExtension() where both renamed to parse.

This also restricts the Exceptions thrown by the parse method, to just
XmlPullParserException, IOException and SmackException (not really a big
victory, but nevertheless a slight improvement).

StreamFeatureProvider is now gone, we simply use PacketExtensionProvider
for stream features.

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

@@ -80,17 +80,10 @@ public class AttentionExtension implements PacketExtension {
      * 
      * @author Guus der Kinderen, guus.der.kinderen@gmail.com
 s     */
-    public static class Provider implements PacketExtensionProvider {
+    public static class Provider extends PacketExtensionProvider<AttentionExtension> {
 
-        /*
-         * (non-Javadoc)
-         * 
-         * @see
-         * org.jivesoftware.smack.provider.PacketExtensionProvider#parseExtension
-         * (org.xmlpull.v1.XmlPullParser)
-         */
-        public PacketExtension parseExtension(XmlPullParser arg0)
-                throws Exception {
+        @Override
+        public AttentionExtension parse(XmlPullParser parser, int initialDepth) {
             return new AttentionExtension();
         }
     }