New packages for organization, closes #460

- flixel.group
 - `FlxTypedGroup`
 - `FlxSpriteGroup`
 - `FlxGroup`
- flixel.effects
 - `FlxTrail`
 - `FlxSpecialFX`
 - fx
 - particles
- `flixel.ui`
 - `FlxButton`
 - `FlxSlider`
 - `FlxBar`
- `flixel.addons.tile` with tile-related stuff
- `flixel.addons.display` with display-related stuff
- `flixel.util.loaders` for the texture packer

- renamed `FlxDisplay` into `FlxSpriteUtil` and moved it into `flixel.util`
- also moved `fill()`, `drawLine()` and `drawCircle()` there from `FlxSprite`