GoKit additions/fixes

Hey Mike,

Throwing a few local fixes your way.

The MaterialColorType is pretty self-explainatory.

setIsTo() for TweenConfig was simply because I was reusing the same tween config for multiple tweens, and I wanted something to match setIsFrom() after I was switching to a new tween that I wanted to be a "to" tween.

Lastly, I added a destructor to the abstract tween class, because I was originally doing this:

```csharp
for (some-loop-here)
{
    TweenChain chain = new TweenChain();

    if (something)
    {
         chain.(....);
    }
}
```

In doing this, I was adding a ridiculous amount of unused AbstractTweens for GoKit to iterate over, and it was killing performance. While I fixed my code to not even call the constructor until I was sure I was going to use it, you may want to put this in as a safeguard for other people.

- Michael