Resolve a DIGEST-MD5 autenthication issue affecting some XMPP servers.

Some servers validate digest-uri, and many prominent XMPP clients return "xmpp/" + realm.
This change adds an option "digest-uri-realm" to emulate this behavior.
Successfully tested on an Openfire server which uses an SRV record (such that host != server-name).
Merge branch 'develop' into imo