Add `FlxGroup#killAll()` and `FlxGroup#reviveAll()`

See https://github.com/FlixelCommunity/flixel/issues/30#issuecomment-26753623

I also added a second commit which points out this new behavior of `FlxGroup#kill()` and just emphasizes the original behavior of `FlxGroup#revive()`. Since we are no longer overriding those methods, adding the relevant ASDoc inside `FlxGroup` would be possible, but seems more complicated than we need to make it. It's easier just to add the information to `FlxBasic` instead.

Should I keep the second commit, or remove it?