Support jqXHR interface on load function

[jacobsvante]

Would be nice to be able to use Thorax's .load() the same way as Backbone's .fetch(). I.e. by using the returned deferred object like this:

myCollection.load().done(function() {
    console.log('yey');
});

I think supporting the complete and error options like the original fetch method would be good practice as well.