Refactoring and code cleanup

[stokito]

Replaced the manual iterator manipulation with a direct iteration on elements() that are anyway used internally:

    public Iterator<Element> elementIterator() {
        List<Element> list = this.elements();
        return list.iterator();
    }

    public Iterator<Element> elementIterator(String name) {
        List<Element> list = this.elements(name);
        return list.iterator();
    }

[guusdk]

I'm generally in favor of this, but if we're going to change the way we implement properties, we should migrate from JiveGlobals to org.jivesoftware.util.SystemProperty. Can you make that part of this PR please?

[stokito]

I don't know yet how to do that so let's do that in a separate PR later.