Rework and improve authentication API and internals

Add

- performSaslAnonymousAuthentication()
- performSaslExternalAuthentication(SSLContext)
- addEnabledSaslMechanism(String)
- addEnabledSaslMechanisms(Collection<String>)

to ConnectionConfiguration.Builder.

Instead of providing a special API call for anonymous authentication,
Smack now has a configuration builder method to set anonymous/external
authentication. This also removes a lot of duplicate code within
Smack.

Also move SASLAnonymous into o.j.smack.sasl.core.

Fixes SMACK-629.

smack-core/src/test/java/org/jivesoftware/smack/DummyConnection.java

@@ -116,23 +116,12 @@ public class DummyConnection extends AbstractXMPPConnection {
     }
 
     @Override
-    protected void loginNonAnonymously(String username, String password, String resource)
+    protected void loginInternal(String username, String password, String resource)
             throws XMPPException {
         user = getUserJid();
         authenticated = true;
     }
 
-    @Override
-    public void loginAnonymously() throws XMPPException {
-        if (!isConnected()) {
-            throw new IllegalStateException("Not connected to server.");
-        }
-        if (isAuthenticated()) {
-            throw new IllegalStateException("Already logged in to server.");
-        }
-        authenticated = true;
-    }
-
     @Override
     public void send(PlainStreamElement element) {
         queue.add(element);