Allow method name instead of callback in `.listenTo[Once]`

This pull request adds support for calling listenTo and listenToOnce with a method name instead of a callback.

    var obj = _.extend({
        mymethod: function(){}
    },Backbone.Events);

    obj.listenTo(otherobj,"someevent",obj.mymethod); // old way
    obj.listenTo(otherobj,"someevent","mymethod"); // new way

Gives nice parity with view events API.