FlxParticle / FlxEmitter improvements

- Added option to decrease scale of particles based on their lifespan
- Added option to decrease alpha of particles based on their lifespan
- Added exists = false to constructor of FlxParticle in order to fix a
bug
Merge pull request #314 from Gama11/dev

FlxParticle / FlxEmitter improvements
fix for issue #318 (Function and Numpad keys caused crashes on cpp target)
little improvement in input classes - they don't use Dynamics and reflection now