TweenManager.isTweening()

*Not sure why there are multiple commits in this pull request, the only file I personally changed is /src/tween/TweenManager.js...*

Anywho, for a game I'm working on, I needed to be able to check any time to see if a Sprite is currently being tweened so I could take action accordingly; I added a function called isTweening() to TweenManager that does just that.

It seems to work quite well, I thought it might be useful to others and could be included in the next version. Here's a usage example:
```javascript
function create() {
    var mySprite = game.add.sprite(0, 0, "blah");
}

function update() {
    // do some stuff that may include adding tweens...

    if (game.tweens.isTweening(mySprite) {
        console.log("I'm currently animating!");
    }
}
```