Added option to pixel-perfect collision check in MouseEventManager

Added parameter ```PixelPerfect = true``` to ```addSprite()```.

Removed the following inconsistent behavior between flash & non-flash targets.
```haxe
#if flash
if (Sprite.pixelsOverlapPoint(_point, 0x0, camera))
#else
if (Sprite.pixelsOverlapPoint(_point, 0x01, camera))
#end
```
now becomes ```if (!PixelPerfect || Sprite.pixelsOverlapPoint(_point, 0x01, camera))```