Add support to change the channel on which requests will be emitted.

Hi.

I've added a patch that allows the model to describe the channel on which the requests will be emitted, Added two tests.

For example :

    var model = new Backbone.Model();
    model.channel = function(params) { return params.url; };
    model.fetch({url: '/one/two'});
    assert.equal(Backbone.socket.path, '/one/two:read');

Seems to work fine.