Make start() of FlxTimer and FlxPath explicit

@gamedevsam @Beeblerox 

Previously discussed [here](https://github.com/HaxeFlixel/flixel/commit/1e6934e2538d1b33cb2fd0148d609144be1a1cf9#commitcomment-8584704).

I feel like this change makes things much less confusing (#1352).