me.Color clone method

@parasyte  Following your refactor of me.Color I would like to also have a clone method added ;)
```
clone: function() {
	return me.pool.pull("me.Color", this.r, this.g, this.b, this.a);
},
```
or something similar in order to make it easier to make variations of a color.

I currently use it like this:
```
var baseColor = me.pool.pull("me.Color", 255, 0, 0);
var shadowColor = baseColor.clone().darken(0.5);
```