SASL Proxy Auth support

This adds the ability to provide a distinct authorization identifier for use
by SASL mechanisms. Not all SASL mechanisms support this operation, in
particular CRAM-MD5.

Both the javax and provided SASL implementations are extended, and an authzid
parameter added to the authenticate method.

The authorization identifier is passed as a EntityBareJid in order to assure the
correct form.

Resolves SMACK-677.

Minor-Modifications-By: Florian Schmaus <flo@geekplace.eu>

smack-bosh/src/main/java/org/jivesoftware/smack/bosh/XMPPBOSHConnection.java

@@ -219,7 +219,7 @@ public class XMPPBOSHConnection extends AbstractXMPPConnection {
     protected void loginInternal(String username, String password, Resourcepart resource) throws XMPPException,
                     SmackException, IOException, InterruptedException {
         // Authenticate using SASL
-        saslAuthentication.authenticate(username, password);
+        saslAuthentication.authenticate(username, password, config.getAuthzid());
 
         bindResourceAndEstablishSession(resource);