Set function context in easyXDM.Rpc

Hello! Great library!

I have created the ability to set context for the `local` functions exposed in an easyXDM.Rpc instance. 
You can set context by setting a new `context` config parameter in the channel configuration.

    new easyXDM.Rpc({
    ...
        context: window,
    ...
    }

The reason I needed this functionality is because I have a Backbone model that is wrapping the Rpc object. I wanted to have control over my functions from the Backbone model itself, plus I wanted my functions exposed in the Rpc object to be easier to test.

    providerRPC = Backbone.Model.extend({
        defaults: {
            xdmrpc: null,
            local: '/js/easyXDM/name.html',
            swf: '/js/easyXDM/easyxdm.swf'
        },
        initialize: function () {
            this.set('xdmrpc', this.createXDM());
        },
        createXDM: function () {
            // this backbone model
            var me = this;
    
            return new easyXDM.Rpc( {
                context: me,
                local: me.get('local'),
                swf: me.get('swf')
            }, {
                remote: {
                    updateOrder: {}
                },
                local: {
                    startup: me.startup
                }
            });
        },
        startup: function (data, success, error) {
            // this == the backbone model
            this.authenticate(data);
            return true;
        },
        authenticate: function (data) {
            alert("You may pass!");
        }
    });

Thanks!

Nate
Is `scope` actually used anywhere? If not, we should probably just drop this, and rely on callers binding the functions  if they please.