Added defaultScope functionality

Added defaultScope functionality, as per this forum post:

http://forums.greensock.com/topic/9938-default-scope-for-events-and-callbacks/

Demo code:

```javascript
function Scene()
{
    // callbacks
        var props = 
        {
            defaultScope        :this, 
            onStart             :this.onStart,
            onUpdate            :this.onUpdate,
            onRepeat            :this.onRepeat,
            onComplete          :this.onComplete,
            onReverseComplete   :this.onReverseComplete
        }

    // animation
        this.tl = new TimelineMax(props);
        this.tl
            .to(this, 0.2, {value:1})
            .call(this.callback)
            .addPause(0.2, this.onPause);

    // resume after pause by moving mouse
        $('body')
            .one('mousemove', $.proxy(this.resume, this) );
}

Scene.prototype  =
{
    // properties
        tl      :null,
        value   :0,

    // events
        onStart:function()
        {
            console.log('start: ', this);
        },

        onPause:function()
        {
            console.log('pause: ', this);
            console.log('move mouse to resume animation...');
        },

        onComplete:function()
        {
            console.log('complete: ', this);
        },

        onRepeat:function()
        {
            console.log('repeat: ', this);
        },

        onReverseComplete:function()
        {
            console.log('reverse complete: ', this);
        },

        onUpdate:function()
        {
            console.log('update: ', this.value);
        },

    // callbacks
        resume:function()
        {
             this.tl
                .reverse()
                .repeat(1); // add a repeat
        },

        callback:function()
        {
            console.log('callback: ', this);
        }

}

console.clear();
new Scene();
```

Output:
```
Scene

start:  Scene {tl: TimelineMax, _gsTweenID: "t31", value: 0, onStart: function, onPause: function…}

update:  0
update:  0.6459750000007792
update:  0.8479000000011634
update:  0.9743999999994645
callback:  Scene {tl: TimelineMax, _gsTweenID: "t31", value: 1, onStart: function, onPause: function…}

pause:  Scene {tl: TimelineMax, _gsTweenID: "t31", value: 1, onStart: function, onPause: function…}

move mouse to resume animation...
update:  1
repeat:  Scene {tl: TimelineMax, _gsTweenID: "t31", value: 1, onStart: function, onPause: function…}

update:  0.9600000000000728
update:  0.8400000000002911
update:  0.5904000000019092
update:  0.36000000000116417
update:  0
update:  0
repeat:  Scene {tl: TimelineMax, _gsTweenID: "t31", value: 0, onStart: function, onPause: function…}

update:  0.9600000000004366
update:  0.8400000000010187
update:  0.6339750000005284
update:  0.36000000000261956
update:  0
reverse complete:  Scene {tl: TimelineMax, _gsTweenID: "t31", value: 0, onStart: function, onPause: function…}

```