Cleaned-up the Bird Entity update function

I hope you don’t mind :
- the res variable was not declared locally
- the previous tween was not locally declared and was potentially
overwritten by the new one
- removed the unnecessary manual animation frame switch (this is
automatically done when calling the parent function)
- removed the “me.Vector2d” object creation just to increment pos.y
(unnecessary object creation stressing the GC)
Merge pull request #16 from obiot/gh-pages

Cleaned-up the Bird Entity update function