fixed opacity setting not being read for TMXObjectGroup
fixed the `collide`/`collideType` functions when using `entityContainers` as child objects
changed `me.EntityContainer` default size to `Infinity`, to ensure they are always "visible"
[#59] added the (optional) possibility to create child `entityContainer` correspondingly to the TMXObjectGroup defined in Tiled

This is configurable through the `me.game.mergeGroup` setting with
default value being true (will create all object under the world root
container)

Also fixed object deletion throught the `me.game.remove` function.