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
Test = function()
{
    // callbacks
        var props = 
        {
            defaultScope        :this, 
            onStart             :this.onStart,
            onUpdate            :this.onUpdate,
            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) );
}

Test.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);
        },

        onReverseComplete:function()
        {
            console.log('reverse complete: ', this);
        },

        onUpdate:function()
        {
            console.log('update: ', this.value);
        },

    // callbacks
        resume:function()
        {
             this.tl.reverse();  
        },

        callback:function()
        {
            console.log('callback: ', this);
        }

}

console.clear();
new Test();
```

Output:
```
Test

start: Test {tl: TimelineMax, _gsTweenID: "t64", value: 0, onStart: function, onPause: function…}

update:  0
update:  0.2603999999996832
update:  0.37589999999978807
update:  0.4743750000000495
update:  0.5643999999998919
update:  0.6459749999997646
update:  0.7243749999998448
update:  0.7884000000000189
update:  0.8439749999999245
update:  0.8943749999999704
update:  0.9323999999999161
update:  0.9619750000000027
update:  0.9843750000000142
update:  0.996399999999993

callback: Test {tl: TimelineMax, _gsTweenID: "t64", value: 1, onStart: function, onPause: function…}

pause: Test {tl: TimelineMax, _gsTweenID: "t64", value: 1, onStart: function, onPause: function…}

move mouse to resume animation...

update:  1
update:  0.9957750000000151
update:  0.9817750000000178
update:  0.9600000000000728
update:  0.9271000000000712
update:  0.8877750000001661
update:  0.8359750000001602
update:  0.7791000000000278
update:  0.7137750000001557
update:  0.6400000000003138
update:  0.5511000000002834
update:  0.45240000000023894
update:  0.35197499999998905
update:  0.24310000000018994
update:  0.12577500000042097
update:  0

reverse complete: Test {tl: TimelineMax, _gsTweenID: "t64", value: 0, onStart: function, onPause: function…}
```