addConnectionPlugin fix

[Liam]

addConnectionPlugin should really create the plugin constructor:

    addConnectionPlugin: function(name, ptype)
    {
        var ctor = function() {};
        ctor.prototype = ptype;
        Strophe._connectionPlugins[name] = ctor;
    }

Plugin instantiation in Connection constructor:

    for (var k in Strophe._connectionPlugins) {
        if (Strophe._connectionPlugins.hasOwnProperty(k)) {
            this[k] = new Strophe._connectionPlugins[k]();
	    this[k].init(this);
        }
    }